When I was twelve, I had been a little constipated and finally went to the bathroom and filled the toilet bowl with blood. Scared me and my mom to death. Apparently, I had ruptured an internal hemorrhoid (blood vessel) and it bled. It only happened that one time. But sometimes when I start my period, it will feel like I need to poop and when I go, I'll start while I'm on the toilet. Could you be starting your period?