I've been feeling nauseous lately?

I've been feeling nauseous the last couple of days. I feel a little better if I sit a certain way or if I walk around, but I usually feel sick, although I haven't thrown up yet. If I eat something like a poptart, I feel like I'm going to throw up. I can only eat light foods, and just a little at a time. Just thinking about or seeing heavier foods makes me feel sick. I've stayed home from school the past couple of days and missed a midterm, so I know I should go tomorrow, but I don't want to go if I still feel sick. If I don't feel better soon, I'll be going to the doctor. Does anyone know what this might be, and if I should go to school tomorrow?