The work up of GI bleeding is based on many things, such as your age, your symptoms, the type of bleeding, your history and physical, etc. While labs and imaging can help narrow down the source, the best diagnostic testing would likely be colonoscopy and/or upper endoscopy. GI bleeding should always be taken seriously. While it may be something as simple as an anal fissure, or hemorrhoids, there could also be a more complex and serious problem such as bleeding diverticulitis, inflammatory bowel disease, colon cancer, or gastric ulcers. The best way to differentiate between these is to start with a colonoscopy and or upper endoscopy (Esophagogastroduodenoscopy).
Despite your concerns regarding colonoscopy, given the limited information, I would agree with your doctor
that you would benefit from a colonoscopy. You should talk to your doctor regarding your reservations of a colonoscopy and why you don't want to have it done. Perhaps understanding the procedure and talking about it more with your physician will ease your mind.
Gastroenterologists, general surgeons
and colorectal surgeons
are the best specialty physicians to go to for a colonoscopy. The sooner you do a colonoscopy the sooner you may have a diagnosis. Speak to your doctor about the procedure before undergoing the colonoscopy.