Hello, I'm Dr Gary. I've been practicing veterinary medicine since 2007. I look forward to helping with your questions/ concerns.
It sounds like she may have one of a few things. She could have some type of trauma. She could have been hit by a car or fell from a tree. in that case, we may have an injury or pelvic fractures. We see many cats with pelvic fractures that have a hard time walking in the rear legs.
It may also be an infection that she picked up. We can see a fever lead to weakness and difficulty walking. If that's the case, we may need some anti-inflammatory meds and antibiotics to help get this resolved.
Lastly, it could be a toxin that she got into. That could lead to metabolic disease (kidney
or liver injury) or electrolyte abnormalities.
I would get her seen and get some bloodwork run. I would also probably get x-rays of the spine and pelvis. From there, she can be treated and get her back to normal. I do think there is a good chance of a full recovery here.
I hope this helps, let me know if you have any other questions.