Too much chocolate?
J’ai mal au coeur.
zhay MAH low KUHR.
I feel sick at my stomach.
Remarkably, this phrase has nothing to do with your heart! Even though the word coeur is a major player in the sentence. If J’ai mal à la tête (zhay MAH lah lah TETT) means I have a headache or My head hurts (it does mean that), you would think that J’ai mal au coeur should mean My heart hurts. But this is one of the ways to say that you feel nauseated.
You can also say J’ai mal à l’estomac (zhay mah lah less-toe-MAH) to say I have a stomachache or My stomach hurts. But J’ai mal au coeur is more specific, because you are explaining in what way your stomach hurts.
I hope you never have to say any of these phrases!
You may also notice that in French we don’t say my stomach, head, etc. hurts. Basically, you are saying I have a pain in my stomach, head, etc. Since there is no way that you can have a pain in someone else’s stomach, there’s no need for the possessive. Of course it’s in your stomach! So the French, very logically, simply says the stomach, the head, le coeur, not mon coeur.