What causes constant clogged/congested head feeling?

I know I get enough sleep (more than usual, because I always have this drowsy feeling in my head and I try to sleep it off, but it doesn't go away). And I don't believe it's caused by allergies, because I've tried several allergy medicines and they don't get rid of my symptoms. I don't really feel sleepy, I just don't feel like doing anything because my head feels clogged up. It feels like there is pressure in my upper head.