Colonoscopies are very accurate, the GI doc is looking right at it during the procedure. A tumor would not present your symptoms. I would eat yogurt enriched with probiotics, eat plenty of fiber and drink water. But I would also see a different Gastroenterologist, just to make sure something else isn't going on since things haven't improved. This is their specialty, and it sounds like your first GI just gave up instead of thinking outside the box.
You need peace of mind and to be able to put this to rest. I know it's very frustrating and scary, so get a second opinion and go from there. Take care.
Thanks for the reply Mammo. I know they are quite accurate, it just scares me every time I see blood in my stool, and they don't know what's going on. Given my age, and a clean colonoscopy and blood tests, I realize it is very unlikely I have a tumor, but it's something that it always in the back of my head. Thanks so much for the reply, I'll get a second opinion right away.
Have they checked you for hemorrhoids as well? If the blood is bright red, this is often the culprit, the brighter the blood the lower the bleed, so a colonoscopy would have caught anything down low. Take care and let us know what you find out.