"I've got an upset stomach" (polite) "I've got a stomach bug" (polite and vague) "I've got a dodgy belly" (informal, implies but does not state you have diarrhoea) "I've got the runs" (slightly crude, inappropriate for some situations) "I've got the shits" (very crude, inappropriate for most situations)

Upset stomach is a good one. Another option that I like to use is: "My stomach is not feeling well/good". (^_^)

"Something I ate disagreed with me" is a personal favorite. "I had/got a stomach bug" simply means your stomach didn't feel very good for a while (normally less than a day). Among native English-speaking friends (this is kind of impolite, but can be funny) you can refer to diarrhea as "the runs" or as "the shits". The more you know...

Im having contractions

Thank you all for answering my question!