I am not very "regular" with my stools so I have a bloody stool every couple months. It happens after a hard stool and lasts about a day or two then it goes away. If your bleeding is happening whether or not you are passing a stool and it lasts more than a few days then I would also suggest having doctor check it out.
could also be colon related. Have a colonoscopy and check for colon cancer
Is the blood in the stool or just the rectum? If it's the latter it could be perhaps a hemroid located just inside the rectum that cannot be felt. I'ts common for a large stool or hard stool passing to irritate those tiny little vessels surrounding the rectum.