Actually, diarrhea with menstruation is pretty common in the general public, colon or not. They believe it is due to the prostaglandin production at that time. The same thing that causes the cramping. The production of this hormone varies from person to person, and can change even within the same person. Hopefully, this is not a permanent change for you and will settle down with time. If not, you can increase your dose of Imodium or lomotil during menses to help control it.
NSAIDs like Motrin block prostaglandins, but some people with IBD can't tolerate it. If you are one of the lucky ones that do tolerate it, it is safe for short term use.
Jan