Sounds like a hemorrhoid to me.. is it itchy? Or does it burn? You posted in the colon cancer fourm, but I really don't think it is cancer.. How long have you had it?

I agree with meblonde...
Sounds like a hemorrhoid, which is a vericose vein of the rectum.
Try swabbing on witch hazel (found where hydrogen peroxide and alcohol is sold) after using the restroom and inbetween. That should minimize the pain and help to heal it quickly.