RebYidd23,
I have occasionally felt ill in the day or two following a fast (fatigue, malaise), and I realized that for me it had little to do with the food that I ate before or after the fast, and much more to do with hydration. The summer fasts especially can be very dehydrating, and I have found that I continue to be dehydrated for a day or two after the fast. So I drink water more frequently for a few days, and that largely solves the problem. Also, I’ve found that eating too fast and drinking too little water when breaking the fast can also lead to gastronomical upset.
Mammele,
For sure! I step down my caffeine intake in the days before Yom Kippur, Shiva Asar bBTammuz, and Tisha B’Av to try and avoid the monstrous headache. For the other fasts, I can wake up and get a cup of tea or coffee in before it starts.