What you describe is not common. That said there were men in one of my support groups who clearly mentioned that their MD noted that radiation can cause the lining of the bowel to become inflamed - leading to loose and watery bowel movements.
In my case radiation treatment inflamed some hemorrhoids. Fortunately it cleared up in a few weeks. You will mostly likely want to check back in with your current or different medical professional regarding your situation. Dennis(ProstateCancer.net TEAM)