Your cat might be suffering from hind limb paralysis. It occurs due to some accident and damage to spinal cord. Tail is actually the extension of spinal cord so when it gets some serious injury, it results into this problem. You have to tell this situation to vet so that he can start its treatment according to actual problem. Provide it comfortable environment and let it take rest. Best of luck.

 I don't have any advice for you, I'm very sorry, but it sounds like you need to take it to a vet. She may need help that you yourself cannot provide her.

 Hi, I have exactly the same problem and none has any idea what is wrong, without test but the vet doesn't know what tests to do. My kitten is only 6 months and it came on very suddenly, he doesn't seem to be in any pain just distressed. If you find out what is wrong please let my know.