There are can be different causes for rectal bleeding.
When the blood is bright instead dark, is more obvious that the bleeding orginates from lower parts of the colon and rectum.
Abdominal pain, diarrhea, and bloody diarrhea (diarrhea mixed with blood) are symptoms of Ulcerative colitis, Crohn's colitis (inflammations of colon), and some bacterial and viral infections (they inflame the colon, too)
There are some other causes, too, like diverticulitis, Meckel's diverticulum, Abnormal collections of enlarged blood vessels under the lining of colon.
For how long you have abdominal pain?
Do you ahve some other gastrointestianl complaints?