What causes blood in stools?

Many reasons: There are many causes of blood in the stool. This is reason for evaluation by your primary care physician or in cases with sigificnt bleeding to get evaluated at the er. It can be from simple fissure of the anus, straining and constipation, hemorrhoids or more seriously ulcers polyps or lessions in the intestinal tract. Untreated can be fatal or at minimum lead to significant anemia.

Bleeding: Blood in the stool may come from the upper digestive tract ( stomach, duodenum, small intestine or proximal colon. Usually this blood gives a black color to the stool and is associated to upper abdominal pain, nausea or vomit or "coffeeground"type vomiting. Bleeding from hemorroids, or diverticulum usually is fresh bright red blood from the rectum. Cancers may cause either one but usually less.

Hemorrhoids: Usually but can be many other things including infection, inflammatory bowel disease and tumors, both benign and malignant. See a colorectal surgeon sooner rather than later.