I don't have IBS anymore, but I used to find prilosec to work wonders at stopping that over-working-tummy kind of stomach ache. If it is an acid sort, try one of the acid-blockers, such as zantac(?)(ranitidine). Both are available otc now I believe.
If it's a long-term problem you might consider re-balancing the flora in your gut by taking a probiotic. It's basically like the live culture you find in some yogurts, so you can eat that instead. The idea is to take a little every day for awhile. A fluid probiotic I've used is called Nature's Life, and just a slug of that a day works. Tastes like kefir, a sweetish fluidic form of yogurt.
Hope it gets better soon.