Welcome to HealingWell!
Have you had a colonoscopy to confirm diverticulitis and rule out IBD & cancer? What about
a stool test? It seems as though your doc thought you had some sort of parasite or you wouldn't have gotten Flagyl (unless I've been misinformed about
Ribbon-like BMs are not uncommon for IBSers. However, it can also be from inflamation of the intestines and narrowing of the walls.
It's important you get looked at by a GI if you haven't already.