Is pizza ok to eat if you previously had an upset stomach yesterday?

I'd wait another day. The problem with recovery is you often have cravings for stuff your system may not be ready for. Any fatty food, cheese, salsa, fried food, require increased digestive work & may trigger renewed sx. Steamed rice, noodles, macaroni without sauces, spaghetti, simple breads are all well tolerated during recovery from a GI disturbance.