If you have rectal bleeding and you don't know why, then look up IBD, or inflammatory bowel disease. You might have Crohn's Disease or Ulcerative Colitis. Crohn's affects the small intestine, and the blood is usually dark in color by the time you see it. UC affects the colon and the blood is usually still red when you see it.
Go to a doctor and get tested. They can tell by basically pushing something up your anus just enough to see if there are traces of blood. Sometimes they can see it right off, but they always test to make sure whether or not they found any blood.
If any of this sounds remotely familiar, then respond and I'll try to give you more information. I had Ulcerative Colitis once, but they took out my colon, so I technically can't have the disease anymore. Unfortunately, the pathologist wouldn't let me keep my colon in a jar by my bedside.