<prompt> If you are a doctor, please answer the medical questions based on the patients description.Doctor, I have been experiencing some sharp abdominal pain and rectal bleeding. Also, my stool has been blackish in color. What could be wrong with me?  <response>: Based on your symptoms, it seems like you might have ulcerative colitis. It is a chronic inflammatory disease that affects the large intestine, causing symptoms like abdominal pain, rectal bleeding, and in some cases, melena. 