The bleeding could be coming from further up the gastro tract that a sigmoidoscopy doesn't get to. Perhaps she should get a colonoscopy, and if that is clear, an upper endoscopy to look at the small bowel. I was on a contraceptive pill for some 10 yrs, and although I have Crohn's, had no bleeding until some 20 yrs later, so don't think the pill is the cause. She should check with both her gynae and gastro to find a proper diagnosis. Bleeding from the rectum should never be ignored.
It could be an anal fissure or hemorroids (sp?) as it was in my case. A colonoscopy should be able to verify this. Sometimes even just a rectal exam can verify this. Diverticulitis (sp?) can also cause this. She definitely needs to follow-up with a gastroenterologist because bleeding from the rectum should never be ignored. Most times it turns out to be hemorroids or a fissure. Don't settle for an unknown cause to the bleeding. Have her bowel movements changed? (i.e. size, color, frequency). These things can help them with diagnosing the cause. It may be embarassing to have to answer some of the questions, but better safe than sorry.