When you eat Mexican food and it causes a stomach ache.
Person 1: *Knocks on bathroom door.* Are you about done in there?
Person 2: I have a bad case of Mexican stomach, so it will take a while.
The act of pouring hot sauce (preferably tapatio) into a sexual partners asshole and proceeding to rail mentioned hole while screaming various mexican gritos
Friend 1: ay guey my girl asked me to give her a Mexican stomach churner last night and i've never been more horrified.
Friend 2: dude you should've done it. I sprang it on the last chick i was with and it real "spiced" up our sex life!