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Hello. I'm sorry for the long wait. I just logged on and saw this.
It sounds like your cat threw a clot to her back legs. This is the most common cause of rear limb weakness/ paralysis with no overt trauma in cats. It is due to an underlying heart disease. Most cats have no signs of disease before the clot causes acute weakness/ paralysis.
Unfortunately, the chance of survival is fairly low. The chance of recurrence after an episode is also very high. Most owners end up euthanizing when a cat throws a clot like this.
Here is a nice link on the condition with more info:
I hope this helps, let me know if you have any other questions.