sounds like she's just not breathing when she's having intense pleasure. and i think not breathing is different from le petit mort.
le petit mort used to be the term for orgasm during the Victorian era and probably earlier. they found that the dilation of your eyes when you have an orgasm is the same as when you die. so they thought that you were dying every time you orgasmed. this is why i get a kick out of reading Victorian literature. any reference to a woman thinking she's dying is an erotic scene (read
The Awakening). now it's reversed: they think that when you die you have a huge orgasm. something they look forward to.
anyways, it could be a problem if she goes out cold for too long *and* is not breathing. brain damage due to asphyxiation=not good. this is not a big concern like autoerotic asphyxiation is because she's not forcibly suffocating herself. if you had said, she asks you to strangle her until she blacks out, then i would be worried. people with this fetish build a tolerancy to how much air they can cut off and then they sometimes go too far and suffocate. for a long time, they thought autoerotic asphyxiation was just a bizarre way to commit suicide. then they discovered it was actually a dangerous fetish that sometimes led to death.
thankfully, though, your body won't let you suffocate yourself. try holding your breath until you suffocate--it just doesn't happen. the only thing that will come of that is maybe passing out. so, if she blacks out for only a little, it's probably not a health concern and you should just pat yourself on the back.