That happened the last cat I hd to. The Vet said that she was in pain and so we had to euthanize her, her name was midnight. Our family was unhappy for a long time. (we got a new kitten named Nikki a couple monthes ago.) The Vet said that it happen because she was the runt of the litter, and runts sometimes have this problem. Well I guess what I am trying to say is that you should just euthanize your cat, no matter how hard it is. I'm sorry and I hope it doesn't happen to you again.
p.s. Midnight was 4 years old to.