Hm, I guess I'm late.
Pain may be from a process in the last part of the colon (maybe trombosed hemorrhoids or abscess, or fissure).
Hm, but you are talking about acidic pain. This may be from bile acids, not absorbed in the small intestine (e.g because of intestinal inflammation) and hence excreted. This would make your stool greenish. There is one drug - cholestyramine - which binds bile acids. Ask doctor if this is for you.
Well, you can tell what was found on sigmoidoscopy.