Here's some info on why your bowels behave differently at different parts of your cycle.
When progesterone is high, which is from the day you ovulate until the day before you bleed, muscles all over your body are more relaxed. Bowels move more slowly. Some women complain that they get constipated.
When the luteal phase is over, progesterone crashes and uterine cramps might begin at the start of your cycle. Any pelvic cramping is going to stimulate your bowels - lots of women (even without IBD) get diarrhea at the start of their cycle. A DR once told me that lots of prostaglandins are released into the pelvis during these kind of events, and prostaglandins are irritating all over. NSAIDs are anti-prostaglandins I think, and that's why you're told to "take Motrin" if you have bad period cramps.
By day 4 or 5 of a normal menstrual cycle, the pituitary gland is in gear sending messages to the ovaries to build an egg - estrogen starts to rise and progesterone is very low, so bowel activity is faster.
Post Edited (kazbern) : 5/10/2010 12:33:57 PM (GMT-6)