Get seen. This could be any number of things: allergies, sinus infection, nasal polyps and more rarely a tumor of the nasal cavities. See your local ENT for a complete head and neck exam and nasal endoscopy.
Chronic sinusitis. The most common explanation would be chronic sinus disease often the result of allergic rhinitis. Nasal polyps can also be responsible. Overuse of decongestant nasal sprays can cause these symptoms. Less common causes include prior injury to the base of the skull, tumor. Degenerative brain diseases or medication side effects.