It CAN be allergies. I have horrible allergies, but I don't have an itchy nose, water eyess etc.. I just get really stuffed up and stay that way.
Have they given you any Rx antihistimines and decogestants? If they help you than it is allergies. If they don't help you then you might have sinus polyps. There is not infection, but the polyps cause sinus symptoms, and that would require an ENT to look at you.