Nasal polyps would be most likely. A corticosteroid nasal spray would be one treatment. Also, removal by an ENT doctor, which is who you should see any way.
Allergies are very common and can go undiagnosed, testing by an allergist being needed.
Chronic blocked sinuses are often undiagnosed as well, and in this case an ENT doctor would open the sinus channel.
Those are the most likely causes.
It is not usually serious.
OK, let me know if you have more questions.
Also, please click a positive rating on the rating scale. In fact, a 5 rating squares me with just answer and allows me to provide prompt and accurate service to you, but does not increase your cost. And, I will be here for any follow up needed.