The slugs don't mean that the snake can't breed in and of themselves. Could be caused by the stress of whatever is causing the breathing issues? If it's that, my guess would be she could breed without any issues. If it's just that she tends to retain ovum, then you might run into it happening again. My exotics vet actually has recommended to me to breed ones that have issues like that, since unfertilized eggs can be harder to pass than babies (especially if they tend to have slugs without being bred).