I experience mild pain in rectum when setting but no pain and no bleeding during bm. what could that be?

See a doctor: Rectal pain can be caused by several things, including hemorrhoids, fissures/fistula, tumors. It is best to see a doctor when symptoms like this are present. A gastroenterologist can look in the rectum with a scope and help determine the cause.

More info needed: You may have strained while sitting --possibly associated with constipation--sounds like you need to see your physician and possibly a nutritionist. Good luck.