Why would there be blood in stool ?

Many possibilities: Broadly, bright red blood in stool comes from bleeding low in the GI tract (large intestine or rectum), or sometimes from rapid upper GI bleeding. Common causes include hemorrhoids and diverticulosis, with inflammatory bowel disease, colon cancer, and gastric ulcers being less common but more dangerous. Blood in stool should be evaluated by your doctor.

Possibly hemorrhoids: Most cases of rectal bleeding are from hemorrhoids. It should be evaluated for anything more serious though like colon cancer. See your doctor.