Gee, that's hard to say. Perhaps diverticulitis... but the colonoscopy should have shown that. Go to www.webmd.com and on the right side of the home page, click on the symptom checker. This may help you. If your pain is more severe in the back, I might even think kidney stones... but check out this site and see if you can get any answers.
Also, don't hesitate to call the doctor. That is what he gets paid the big bucks for.