I get anxiety thinking about being in school for long periods of time..how do i get over this?

im in college now and i can do 2 hour lectures the max but i have some classes this semester in the summer that are 3 1/2 hour long and my stomach gets very weak just thinking about it. i get nauseous physcally cause my stomach has always been very rumbly and i get cramps easily so i feel uncomfotable being in a room that long with my stomach problems. is there anything i can do to get over this....medicine to calm me or my stomach? any tips i will appreicate=]