Check out this thread too...
http://www.medhelp.org/posts/Womens-Health/vaginal-bleeding-with-bowel-movement/show/32315
I wrote on that thread what is below but after reading this, I feel like I should throw in some more information. I had PID when I was 18, then also had a miscarriage and a D&C. When I was 19 I got a pos HPV at my pap. They did a cone biopsy on my cervix and LEEP surgery and I never had another pos test. I had my first child at 22 and my second at 31. I also have IBS and am overweight.
I am a married 37yo with a 15yo and a 6yo. This same thing started with me a little over a year ago. About a week or so before my period, when I have a BM I bleed. Sometimes it's a lot, sometimes it's a little, but I always have dull ache cramps to go with it. I also have a tilted uterus. I was on birth control when it started, the SAME birth control I had been on since my son was born, but my obgyn said it was just breakthrough bleeding. I stopped the birth control in August of last year and it is still happening. I have read that sometimes antidepressants can cause it, but the two that I am on (Celexa and Wellbutrin) are on the 'least likely to cause those side effects' list. My grandmother, aunt, and mother all had to have hysterectomies in their late 30's and early 40's so I hope that it is nothing too bad.