Help!! Bad stomach pain!!?

So, I have this really bad stomach pain in my upper and mid abdomen, it's a gnawing pain, and it feels like someone punched me in the stomach. It comes and goes but is made A LOT worse after I eat and I can't eat as much as I want and I can't afford to lose any weight it really hurts and causes nausea. What is it?! And when will it go away?