diarrhea acidity - any way to neutralize it?

I'm going in for a flexible sigmoidoscopy, and I've discovered that it's not the procedure that is painful or uncomfortable, it's the extreme diarrhea you get in preparing for the procedure. I'm going in for this procedure because I have a lot of little cuts and I bleed a lot sometimes, and the acidic nature of diarrhea makes this so painful that I cry in agony and would rather make the doctor angry/not do the procedure and take something to calm it down. The pain when I get diarrhea is to the point where I honestly have only felt pain similar to it after abdominal surgery.
If anyone could suggest foods or ways that I can reduce the acidity or reduce the amount of pain I will be going through when I prepare for the procedure with a laxative and an enema I would greatly appreciate it.
ANY HELP at all would be awesome, considering my procedure is tomorrow and I need to take the laxatives tonight.
Thanks guys...

[boron]

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.