Is diarrhea common in third trimester?

It's somewhat common as the stomach becomes more sensitive, but obviously you should discuss it with your doctor.

Diarrhea is not uncommon in the third trimester, in fact, diarrhea is actually not entirely unusual at any point in pregnancy, although third trimester diarrhea does seem to be more common, especially the closer you get to your due date. It is often nothing to be concerned about. Keep well hydrated to prevent dehydration. Also keep an eye out for other symptoms such as fever to make sure you are not coming down with an illness. If you begin running a fever or experiencing other symptoms that might indicate you are sick, contact your health care provider.