There can be many causes like Allergy, Infection, Trauma, Inflammation Parasites, Cancer of colon or rectum area, Foreign material in colon or rectum, Disease of the anal sacs, Constipation / stool buildup causes lower bowel damage, stool and matted hair block the exit of the bowels or the anus and tissues may lose blood secondary to infection and pressure on tissues. But, still every thing depends on the color of the blood or it is fresh blood or not. Take your cat to pet for a thorough examination. Check if it is constipation. Change its diet to fluid intake.
