I always carry the pepto/immodium in case someone has an upset tummy after a meal...but I find that if we slow down and don't rush right after meals, we feel better overall.
Also, don't drink the water. We never drink anything but bottled and we buy spring water by the gallon to keep in our room.
This is so important, and people forget but it applies to brushing teeth, too. I even brush my teeth with bottled water. If you're really sensitive to the water, remember that most sodas from fountains are made with the local water, too. Buy your pop in bottles/cans.
I personally have found that I just simply cannot eat salad or raw fresh vegtables in WDW. I don't know why, but for some reason if I do, I am up all night and hating myself. I can eat fruit, but not the veggies, especially lettuce.
This is a mega bummer because I'm a salad freak, but once I figured out what was doing it to me, I have been able to ward off most stomachaches. I wonder if it was something they were washing the veggies in or something used in the growing process, never figured that out, just know that salad is a definite don't for me in WDW.
Hope you can figure it out as to what is effecting you, take some meds with you, and feel better on your next trip!