I would suggest you see a physician ASAP. Severe rectal bleeding can be caused by serious conditions.
An endoscopy is suggested, either a flexible sigmoidoscopy or colonoscopy. Possible causes can include hemorrhoids, a mass, tumor, cancer, or diverticulosis.
Without knowing exactly what it is, it's difficult to proceed with the next step.
This answer is not intended as and does not substitute for medical advice - the information presented is for patient education only. Please see your personal physician for further evaluation of your individual case.
Kevin Pho, M.D.
KevinMD.com
Twitter.com/kevinmd