Why can I only breathe out of one nostril?

First of all I am sorry to hear that you are only able to breathe out of one nostril. I am happy to give you some ideas as to what might be going on, but to get the most thorough answer, you will need to see a physician for a thorough history and exam. By the nature of your complaint, I would recommend making an appointment to see an ENT (Ears, Nose, Throat) physician. The differential diagnosis for nasal obstruction is extremely diverse. It can range anywhere from inflammatory obstruction (such as turbinate hypertrophy, sinusitis, nasal polyps, allergic rhinosinusitis, etc), to fixed rigid obstruction (nasal boney obstruction, septal deviation, tumors, etc), to dynamic obstruction (like nasal valve collapse with inspiration). Being that you have obstructive symptoms that go from side to side, I doubt that there is a significant rigid obstruction on just one side, which narrows the differential a little. Also important to keep in mind is that there is something called the nasal cycle (which is normal). There is erectile tissue within your nose on structures called the turbinates and the septum (to a lesser extent) which functions to humidify and warm air as you breathe it in. every 3-4 hours one side of your nose swells, while the other decongests. This may be what you are experiencing. Like I told you earlier, the only way to know for sure is to have a physician take a look in your nose. I hope this helps. Best of luck.