I don't think your stomach probs would have anything to do with anal sex. The only thing I can think of is if you are feeling uneasy about the anal and your nerves are making your stomach upset.

This could definately be from anal sex. You see, some women even with regular vaginal sex can get stomach pains and sick to their stomachs. It depends on how your body is put together and how close your "parts" are. For example, with vaginal, or anal you could be rubbing against your uterus or your bowels and this can make you dizzy, light headed and sick to your stomach. So, depending on your body and how close everything is, anal sex could definately make you sick.
Not to mention it can cause bowel disorders, hemmorroids, it spreads infections, and also a lot of std's are transmitted more easily through anal sex than vaginal. So, you just have to weight the pros and cons for yourself and go from there. Good luck.