At the CS restaurant at AKL, The Mara, Fresca has been available as a fountain drink. That is the only place I've seen Fresca at WDW. Some places have Sprite Zero.
Just a word on a the "zero" drinks - I know they're supposed to be 0 calories, so one would assume they have no sugar in them. I beg to differ. I have Type 1 Diabetes so I only drink diet soda. I've tried a zero product twice now and both times it shot my blood sugar levels up just as if I had drunk a regular soda. So be warned!
Wow! That is really terrible. My pre-diabetic mom drinks Zero products all the time. Are you sure it was the soda and not something else you consumed with it?