Almost Every Time I Eat, 30 -40 Minutes Later I

Almost every time I eat, 30 -40 minutes later I have very loose bowel movements and often uncontrollable. in my left lower abdomen It bubbles and churns and then the diarrhea starts. I had a colonoscopy and dr said I had diverticulitis to eat more fiber. I do that, still does not help. Could it be gall bladder?