upset stomach; the runs
Osaka dialect expression for a weak stomach or diarrhea, often used in a light, self-deprecating way. ピーピー is onomatopoeic for the sound of loose bowels.
I ate too much yesterday, so I've got the runs.
I get an upset stomach easily when I'm nervous.
Compound of お腹 (stomach) and the onomatopoeia ピーピー (sound of loose bowels). The exact origin is uncertain, but it is a characteristic Osaka dialect expression.