Diarrhea and Proctitis. These symptoms are common from radiation therapy to the abdomen and pelvis. These are not the same thing, although chronic diarrhea may cause or worsen proctitis. Diarrhea is when you have frequent, watery bowel movements. It may be accompanied by gas and cramping. Proctitis causes rectal irritation, pain, burning, and sometimes bleeding. It may be associated with having small, frequent formed stools. Your treatment may cause one, or the other, or both.