When you get the stomach ache it is your guts (intestines) that are probably hurting!
I've just done a basic first aid course and your heart is basically between your nipples (if you have pert boobs (or none!) but if your boobs are round your waist then probably look behind your ears!)
I must admit I didn't think that the stomach was that high up, but when you get a 'tummyache' or 'stomach ache' it's usually lower down and everyone (even doctors!) know what you mean.
I certainly wouldn't be embarrassed by not knowing exactly where my actual stomach was.