Well, Diet Soda contains caffeine which is a diurretic. This makes you pee more often and could make you dehydrated. Especially if you have diarrhea.
The BRAT diet includes bananas, rice, applesauce, and toast.
because of the clopiha intocine in the soda to reduce sugar and the by product of that is diareeia
Just personal anecdote - when I was on phase one of the South Beach I did have diarrhea. Lots of veggies, cheese and eggs - no carbs. I think my system has alot of fiber and roughage!!!
Diet sodas in excess can cause diarrhoea, as they contain sugar substitutes and artificial sweeteners such as sorbitol.