The two most common causes, especially in young people, are anal fissures and hemorrhoids. Both of these problems can be caused by constipation. The anus is the opening through which bowel movements pass. Fissures are most often caused by hard bowel movements that tear the skin as they pass through the anus. They can also be caused by prolonged diarrhea or anal intercourse.
Hemorrhoids are swollen veins in the lower rectum and anus.
They may be external, which means they can be seen outside the anus. Or they may be internal, which means they are up inside the rectum and can't be seen without special instruments. Diverticuli in the colon are a fairly common cause of rectal bleeding in middle-aged and older adults. Diverticuli are fingerlike pouches at weak points in the wall of the colon. In this case the bloody diarrhea lasts for 1 to 3 days and then is gone.