Heartworm disease is more prevalent in Florida and the surrounding southern states. I think if you're worried, get your other 2 cats tested (simple blood test). Have the vet do a complete check up too (i.e. checking for a heart murmur). There is a medication called Revolution which you can put on your cat (it's a topical liquid) that is a heartworm and flea preventative. I have seen 2 or 3 year old cats die suddenly at home and discover on necropsy that they had cardiomyopathy. It's always such a shock. So in an 11 year old cat, it is certainly the highest possibility on my list. Again, I'm so sorry for your loss.