The Hershey's with lactitol do it to me, too. Even one. In fact, I just ate one today because I need a laxative. It worked very fast.

Ironically, my digestive tract isn't bothered by the Russell Stovers, which are sweetened with maltitol. There was a thread on this a while back and most people were the opposite--it was maltitol that bothered them and not lactitol.