Why do I feel sick every time I eat?

Every time I eat, no matter what it is or how big or small it is, my stomach aches and I feel dizzy. I sometimes go a few days without even needing to eat, but I get to the point where I really want to eat, even though I'm not hungry, and when I do I just feel even more sick. Even the smell or thought of certain foods makes me want to throw up now.