When your nose suddenly stops breathing well, the cause is usually either a virus (a cold) or a sudden allergy flare-up. Below are clues that can help you tell the two apart. Knowing which one it is makes it easier to know what to expect and how to treat it. 

Clues that point to a virus (a cold) 

If you have been around young kids or people who are sick, a virus is the most likely cause. Colds can feel different from one to the next, so don't assume the next one will feel just like the last. There are hundreds of viruses, and each one can cause slightly different symptoms. 

Viral infections often start in one spot — one side of the nose, the throat, or the chest — and then spread to the rest of the upper airway over a day or two. People often make a lot of clear, watery mucus at first. 

If you are not sure what is causing your sudden stuffy nose, it is safest to assume it is a virus. Treat yourself as if you are contagious and wash your hands often so you do not spread it to others.

If allergies are also a possibility, I usually still treat the patient as if it is a virus, but I may add an antihistamine (an allergy medicine) to the plan.

Clues that point to a sudden allergy or irritant reaction (allergic and non-allergic rhinitis)

If you have been around something you know you are allergic to, a sudden allergy flare-up is possible. A sudden reaction to an irritant (this is called non-allergic rhinitis) is also possible. These reactions usually settle down quickly once you get away from the trigger.
Common non-allergic rhinitis triggers include: 
Perfume, fumes from cleaning products or paint, smoke, and dust. 
Allergic and non-allergic rhinitis usually has a pattern — you may have had short-lived flare-ups like this before. Sneezing, itchy red eyes, no sore throat, and no cough are clues that this kind of trigger is to blame. If you do not get this kind of stuffy nose very often, and you have not been around an obvious irritant, then allergies or irritants are probably not the cause.

Long-term sinus infection (chronic sinusitis)
A crooked wall inside the nose (deviated septum)
Sinus cysts or polyps (small growths)
An air bubble in a normal nasal bone (concha bullosa)

Long-term sinus infection (chronic sinusitis)

People often mistake this for allergies, but allergies usually do not cause pain. Chronic sinusitis can cause many symptoms — stuffiness is the most common — but ongoing sinus pain often points specifically to sinusitis. Symptoms often come and go in waves. Antibiotics often help, as do steroid medicines (a Decadron shot, a Medrol Dosepak, or prednisone). Some people live with mild chronic sinusitis without much trouble. Colds and allergy flare-ups tend to hit harder when you also have chronic sinusitis. The pain can show up over the eyes, between the eyes, in the upper teeth, in one or both ears, or in the back of the head. The location depends on which sinuses are affected.

A crooked wall inside the nose (deviated nasal septum)

The septum is the bone and cartilage that divides the left and right sides of your nose. It is almost never perfectly straight. When it is very crooked, it can have sharp bony "spurs" that press into the side wall of the nasal cavity. This is quite common. In other cases, a wider bulge can press against the side wall. Often, there is no history of an injury to explain it. A crooked septum often blocks one side of the nose, and sometimes both.

A sharp bone spur in the septum can cause pain. The pain is usually on one side, often feels like it is in the side of the face or ear, and gets worse with nasal congestion. A simple home test: spray a decongestant nasal spray on the side that hurts. If the pain eases noticeably soon after, that adds to the evidence that a spur is the cause. There is a specific form of pain in this area caused by a sharp bone spur, called Sluder's Neuralgia.

This kind of sinus pain can come and go. The first treatment is usually steroid nasal sprays, with occasional use of decongestant sprays and decongestant pills. If that does not help enough, you should see a sinus specialist for the diagnosis and a treatment plan. A CT scan, X-ray, or a scope exam (endoscopy) may help confirm what is going on. Long-term pain from this cause often improves with surgery.

Sinus cysts or nasal polyps (small growths)

Cysts and small polyps inside the sinuses are common (the most common is a small cyst in the cheek sinus, called the maxillary sinus). Most of the time they cause no symptoms. Sometimes a cyst or polyp blocks the opening of a sinus, and the pressure that builds up can cause pain. Cysts are usually filled with clear or straw-colored fluid. Once in a while, the fluid in a cyst becomes infected, and that can cause pain near the cyst.

The only way to find sinus cysts and polyps is with an X-ray or CT scan. Even when one is found, it can be hard to know if it is causing the pain — in many cases it is not.

An air bubble in a normal nasal bone (concha bullosa)

This is a fairly common normal variation in anatomy. The middle turbinate is normally a thin plate of bone inside the nose. Sometimes that plate forms a small bubble or air pocket inside it. That makes the turbinate bigger than it should be, so it presses on the structures around it and can cause pain. Think of it like trying to fit 10 pounds into a 5-pound bag.

Pain from a concha bullosa is usually felt between the eyes. It can be on one side or both. Larger ones are more likely to cause pain. Many smaller ones cause no problems at all.

A CT scan is needed to spot a concha bullosa and to help judge how likely it is to be the cause of pain. It can be treated with steroid nasal sprays and decongestants, or, if those do not help enough, with a small surgical procedure.

When sinus pain is your only symptom, it is usually best to look at other causes first. It is uncommon — but not impossible — for the sinuses to be the source of pain when there are no other nose symptoms.

For example, pain in the upper teeth might first be checked by a dentist. Ear pain can come from an ear problem or from the jaw joint (TMJ). Pain between or over the eyes is often a type of headache, even when it feels like it is coming from the sinuses. In patients whose only symptom is pain, I think it is helpful to get a CT scan early, so other causes can be looked into if the sinuses do not explain the pain.

A quick home test can help. If two sprays of a decongestant nasal spray seem to relieve your sinus pain, that points toward a sinus cause. If the spray gives no relief, that does not rule out a sinus cause. Remember, this is a test, not a treatment. Decongestant sprays should only be used for a few days in a row. 

Headaches (with no clear sinus cause)

A crooked wall inside the nose (deviated septum)

Jaw joint problems (TMJ syndrome)

Sinusitis
Headaches

For people who have only sinus pain and no other nose symptoms, sometimes no clear sinus problem can be found. You may have already been treated for sinus infections, allergies, and other nose issues without much success. At some point, a doctor will likely order a CT scan to look at areas they cannot see during a regular exam. In many of these patients, no clear cause is ever found.

When that happens, the pain is often best treated like a headache. Some specialists tell patients, "You don't have sinus headaches." I do not agree with that approach. After all, a normal brain MRI does not mean a person does not get headaches — and the same logic applies here. A lot of this is just word choice, but it matters: doctors should be careful not to make patients feel like their pain is being brushed off.

What I usually say is, "The CT scan doesn't show any infection, blockage, or unusual anatomy. I don't think antibiotics, allergy medicines, or sinus surgery will help. Let's try treating this with headache medicine instead and see if we can take the pain down." At that point I usually try a couple of migraine medicines and refer the patient to a neurologist for a full headache workup. These patients have almost always already been on many sinus medicines from their regular doctor. As it turns out, steroids and decongestants sometimes help headaches even when no clear sinus problem is found.

One reason your sinuses can hurt when no obvious problem is found: the nerves that feed the lining of your nose and sinuses can get irritated or inflamed for reasons we do not fully understand. It may be the same kind of process that causes the nerves around the brain to hurt and lead to tension and migraine headaches.

A crooked wall inside the nose (deviated septum)

The septum is the bone and cartilage that divides the left and right sides of your nose. It is almost never perfectly straight. When it is very crooked, it can have sharp bony spurs that poke into the side wall of the nasal cavity. This is quite common. In other cases, a wider bulge can press against the side wall. Often, there is no history of an injury to explain it. A crooked septum often causes a blocked nose, but not always.

A sharp bone spur in the septum can cause pain. The pain is usually on one side, often feels like it is in the side of the face or ear, and gets worse with nasal congestion. A simple home test: spray a decongestant nasal spray on the side that hurts. If the pain eases noticeably soon after, that adds to the evidence that a spur is the cause. There is a specific form of pain in this area caused by a sharp bone spur, called Sluder's Neuralgia.

This kind of sinus pain can come and go. The first treatment is usually steroid nasal sprays, with occasional use of decongestant sprays and decongestant pills. If that does not help enough, see a sinus specialist for the diagnosis and a treatment plan. A CT scan, X-ray, or scope exam (endoscopy) may help confirm what is going on. Long-term pain from this cause often improves with surgery.

Jaw joint problems (TMJ syndrome — short for temporomandibular joint dysfunction)

I'm not a big believer in TMJ as the explanation for everything. Some doctors think most pain near the temples or ears comes from the jaw joint. There certainly are patients who have this problem, but I do not think it should be the go-to diagnosis for face pain of uncertain cause.

That said, TMJ should be considered if you have pain right over the jaw joint, if you grind your teeth at night, or if you recently had major dental work. If chewing makes the pain worse or brings it on, or if you have already looked into every other cause, it is reasonable to think about TMJ as a source of headaches and sinus-style pain.

TMJ pain is usually around the ear, the temple, or the angle of the jawbone. It is best evaluated by a dentist who has a special interest in this area. It is treated with simple measures first, and then with a bite splint (which you can get over the counter or from your dentist).

Concha bullosa and other small anatomy variations

The middle turbinate is normally a thin plate of bone inside the nose. Sometimes that plate forms a small bubble or air pocket inside it. That makes the turbinate bigger than it should be, so it presses on nearby structures and can cause pain. Think of it like trying to fit 10 pounds into a 5-pound bag.

Pain from a concha bullosa is usually felt between the eyes. It can be on one side or both. Larger ones are more likely to cause pain. Many smaller ones cause no problems at all.

There are other small anatomy differences that can cause sinus pain in similar ways — basically, two structures that should have a small air gap between them end up pressing against each other. Examples include a club-shaped middle turbinate, large "agger nasi" cells, and large "ethmoid bullae" (these are all small structures inside the nose and sinuses).

A CT scan is needed to spot a concha bullosa and to help judge how likely it is to be causing pain. It can be treated with steroid nasal sprays and decongestants, or, if those do not help enough, with a small surgical procedure.

When sinus pain is your only symptom, it is usually best to look at other causes first. It is uncommon — but not impossible — for the sinuses to cause pain when there are no other nose symptoms.

For example, pain in the upper teeth might first be checked by a dentist. Ear pain can come from an ear problem or from the jaw joint. Pain between or over the eyes is often a type of headache, even when it feels like it comes from the sinuses. In patients whose only symptom is pain, I think it is helpful to get a CT scan early, so other causes can be looked into if the sinuses do not explain it.

A quick home test can help. If two sprays of a decongestant nasal spray seem to relieve your sinus pain, that points toward a sinus cause. If the spray gives no relief, that does not rule out a sinus cause. Remember, this is a test, not a treatment. Decongestant sprays should only be used for a few days in a row.

Sinusitis

People often think of sinusitis as a cause of sinus pain, and it is indeed one of the more common causes. It is unusual for pain to be the only symptom of sinusitis, but it does happen. If a single sinus — for example the ethmoid sinus, frontal sinus, or sphenoid sinus — becomes blocked or infected on its own, it can cause pain without causing a stuffy nose or drainage.

Clues that a blocked or infected sinus is causing your pain, even when you have no other symptoms:
The pain is on one side

The pain gets worse when you lean your head down low

The pain started after a cold or sinus infection, even though the other symptoms cleared up

The pain clearly gets better with antibiotics

A CT scan is usually needed to clearly diagnose this kind of problem. Plain X-rays often miss it. If a blockage and infection were big enough to show on a regular X-ray, there would almost always be congestion, a blocked airway, or drainage as well.