I know with deep tissue/Swedish massages your body releases lactic acid into your system. Lactic acid is found in yogurt, sour cream, buttermilk, etc. If those products have upset your stomach before, then it's quite possible that the massage was the cause of your diarrhea.
A massage is like running. Make sure you don't eat 2 hours before and make sure you stay away from sugary foods. It will help ease your stomach a bit.