Benign Polyp Removed, Stools Are Narrow. Had Colonoscopy. Advised To Eat Fiber. What Can Resolve The Issue?

I had a colonoscopy at the end of April, during which a large polyp was removed. It was benign and my colon was otherwise normal. Ever since the colonoscopy, my stools have been narrow. I have never experienced this before and it happens during every single bowel movement since then, every day. My GI doc has o explanation and suggests eating more fiber and taking a fiber supplement. I have been consuming lots of fiber, with no change. I did purchase a supplement today to try, but am concerned that the procedure altered the structure of my colon somehow. What could be a possible cause, and what else can be done to resolve the problem, or should I consider this a new normal?