Severe Physical Fatigue

I'm 18 years old and for a long time every couple of months I seem to hit a few days where my body gets weak to the point where I cant walk by myself without collapsing or lift a simple pitcher of water, I've been to the doctors to have them test me for anemia per request of my mom, but that's not it, does anyone have any ideas on what it could be?

If you think this relates at all for the past 6 days I've had a constant headache that won't go away with simple pain relief medications like alive, and my stomach has been acting strange I'll be very hungry but when I eat I get full after a few bites and begin feeling very nauseous immediately after I eat for anywhere from one to two hours.