While running does cause very minor blood loss from the intestines (and broken capillaries in one's feet--things that can contribute to anemia in runners), I have been running for almost 4 years...trained for and run 3 marathons. I've never had noticeable rectal bleeding. Girl, please get into the ER ASAP! Bleeding (especially with abdominal discomfort) should always be taken seriously, even if it ends up being benign.