This was one of the side-effects that the Rad. Oncologist said were stress-related, not radiation, no way, no chance. My GP says they are radiation pure and simple - seen often as he has patients return to him after being closed out by the RO.
I went from 2 bm per day to at least 4, sometimes more. Not loose bowels, just frequent need. Alf posted a good site in the UK a couple of months ago that described my situation to a T. I had a serious previous battle with colon "issues", so live on stool softeners, which likely make it worse.
Rule 1 - don't fart after RT unless already seated on the porcelain throne.
Rule 2 - see rule 1 after checking seating.
For me, rule 1 kicked in about 6-7 weeks into IGRT.