My cat had similar thing...we took him to emergency vet and were told it was a probable DVT. This is a blood clot that broke free to his hind quarters. It is common in older cats. He was happy and healthy up until the sudden onset of this. We did have to put him to sleep because he was in such pain and were told he could not recover from this and would continue to suffer until he died on his own. Hardest thing we had to do but did not want him to suffer needlessly...
