I may be too late in suggesting this, but your vet could do an autopsy. My Whitney (standard poodle) died one day when we were all at work and had been in excellent health. The vet said it was most likely a heart attack but that he could do an autopsy to find out for certain. We didn't have the heart for that so said no. It might help to shed some light on what happened to your kitty, but it is also a very horrible thing to have to consider doing to a beloved pet.