Ouch!

I have been trying to get my tummy back in shape but every time i do sit ups or anything, that night i feel sooooo sick. it doesn't feel like muscle soreness, just like a really bad stomach ache. is anyone else having this happen? I can handle sore muscles, but i really feel like i am gonna wretch every night and end up going to bed early. i feel like i am being such a baby but it's really getting old ;p