I get something similar, but I've also been diagnosed with depression and an anxiety disorder that causes my Irritable Bowel Syndrome (oh the fun :|). Your stomach is very sensitive to stress and anxiety.
Someone above mentioned Imodium... please only take this if you're prepared to be constipated for the best part of a week afterwards. Imodium is best for diarrhoea that's causing you a huge dehydration risk, as it stops it fairly instantly, but keeps it that way for a while! Your best bet is to settle your stomach with something like Pepto Bismol as it is far kinder to your stomach lining, and the only usual side affect is slightly blackened stools a day or so later. Also, Diocalm tablets are effective, although they take a while to kick in, but they have less side affects.
With any diarrhoea please make sure you have plenty of fluids afterwards, it dehydrates you very quickly. I hope you manage to do something to make it easier when you're stressed, I know it's not fun!