Diarrhea is nobody’s friend, whether you're pregnant or not; but for many moms-to-be, it's (unfortunately) a pretty common visitor late in pregnancy. Regardless of whether or not you’re having a baby, diarrhea's usually caused by changes in your diet; so first start paying attention to what you’ve been eating lately. Try cutting out foods that will leave you running for the bathroom (like prunes) and substituting them for foods that will "bulk up" your poop (like bananas). Most importantly: don’t forget to hydrate. You need to replenish all the liquid you’re losing.
But if you’re at the end of your third trimester and approaching your due date, then your diarrhea is most likely a pre-labor symptom. Think of it as your body's natural way of cleaning out and prepping for baby’s big arrival. Of course, if your symptoms become severe, you should see your doctor immediately.