For 2 years I've had a constant pressure in my head that only goes away for short periods of time after meals, what's wrong?

Type of headache. You have chronic headaches, which simply get better with a carbohydrate load. If you haven't seen a neurologist, do so. These are pretty much always benign.
Don't know. Could be nothing or metabolic. Talk to your pcp. Get a scan and lab work up.