I noticed I get diarhea around my period too. I thought this was very odd at first and talked to my doctor about it, but for me she said it was normal. I only have an ileostomy with all of my colon left and apparently there is still mucous in the colon that needs to be expelled. The cramps from having my period seem to make it all worse.
Dx/ed October 2003,
April 2004 - Surgery - 2ft of small bowel removed
Feb 2007 - Surgery again and a temporary ileostomy
Present - on Remicade waiting for surgery to reverse the ileostomy - Health Care System in Canada sucks - wait list is 6+ months.