Take your cat to the vet immediately. Sometimes if a cat has an enlarged heart, a blood clot can happen in the back that not only causes indescribable pain and back leg paralysis the cat also looses all control of it's bladder and doesn't know or care to know where it is all it can concentrate on is the indescribable pain. If this is the case then most humane thing to do is to put it to sleep. I am not a vet, but I have had experience with senior cats.
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