What causes an anus to bleed during bowel movement?

Hemroids: This is the most obvious answer but not the only correct answer. Hemroids are enlarged blood vessels that can be fragile and bleed easily and also painful around the anus both externally and internally. But you could have a colon problem and the blood could be coming from there. See your doctor, he may want you to have a colonoscopy where they use a small camera ango up the colon and can check.