Caffeine may stimulate your gastrointestinal tract. That can lead to an upset stomach, nausea and diarrhea. Coffee, for example, is acidic, which can irritate your stomach lining and cause abdominal cramping. Its laxative properties (likely due to its acidity) can also give you diarrhea if you drink a few cups a day (even if you drink decaf). If you don't want to give up coffee, drink richer varieties like French roast or espresso, which are less acidic and contain less caffeine.