Quoting Nicoℓe ♥:" Ill be 21 weeks pregnant tomorrow, and its happend twice in the past 2 weeks where I was trying to use ... [snip!] ... both times I was in the middle of taking a bowel movement. Has anyone else expierenced this before? Do you know why it happens?"
It's just the baby moving around if she was on top of your bladder or if you have to strain yourself down there.. this type of pains are expected throughout your whole pregnancy. I wouldn't worry unless you're seeing any type of blood or the pain is persistent after a few moments.