Are you going to get the mother fixed? If you have many feral cats and kittens around you should. I'm not trying to sound harsh but feral cats decimate small game animals because they are such accomplished hunters. There are very few predators to pose a threat to them and they overpopulate an area rather quickly.
I sorry for your loss as I love animals and hate to hear something like this, I hope that you feel better soon and at least give your vet another chance.
( I know I sound like an A**h*** but I don't mean it that way )