Hi, TC,
You do not say how old your cat is, but I suspect that she is not in the first bloom of youth!
There are several possible explanations of the signs you decribe - though most of them should have showed up on x-ray.
Among those which would not I would include a thrombosis of her iliac veins, not uncommon in cats, and starving the muscles of the hind legs of blood. Some sort of nerve damage following injury is also a possibility.
As and if further signs develop your vet will be able to narrow down the diagnosis.
W