Could be dangerous. There are many causes of blood in the stool, ranging from benign to malignant (cancer). Bloody stools can be caused by hemorrhoids, colitis, diverticulosis, colon cancer, and more. If a person has blood in his or her stool, it should be checked out by a doctor as soon as possible.
See your doctor. While it may be as simple and benign as a small tear in the anus due to passage of large hard stool, it could also be a sign of something more serious. Possibilities include infectious colitis, colon cancer, meckel's diverticulum, inflammatory bowel disease, ischemic colitis. See your doctor for a complete history and physical exam, and to determine if further testing is necessary.