Thank you for the reply.
There are a couple of reasons that her walking may be abnormal. Even though it seems as if it should be somethings that produces symptoms over time, it is not uncommon for degenerative joint disease (DJD - or arthritis) to present acutely. When this happens, a cat will appear wobbly or less sure footed as they are not putting full weight on the hindlimbs. Pain is a very difficult thing to judge, especially in cats as they are so good at hiding illness. Sometimes the only way to tell if their symptoms are due to the pain of DJD, is with monitoring response to a trial of pain medication. Of course, xrays will sometimes help support that diagnosis if there are degenerative changes seen in the hips or knee joints.
Other causes of acute hindlimb weakness would include vascular disease like a clot to the large vessels supplying the hindlegs. When these occur, they usually produce a more painful consequence and total loss of function of the legs. If there were a partial clot, it could produce lesser signs, but that is not usually the norm.
Finally there are neurologic causes. Anything that irritates the spinal cord or the nerves supplying the back legs could be involved. Disc disease, as seen in humans and dogs, is not a common in cats, but can occur. More often the nerves would be affected by inflammatory conditions or tumors. Vascular compromise to the nerves (like a stoke) is another possibility, but difficult to prove without advanced imaging.
One other thing to consider when a pet experiences sudden loss of balance and ataxia (weak and uncoordinated walking) is vestibular disease. The vestibular system is one that controls the sense of balance and is located in the brain. The vestibular nerve, which exits the brain travels through the inner ear, so an inner ear infection can be one cause of irritation to the nerve and can lead to vestibular symptoms. A brain lesion, like inflammation or even a tumor can also be a cause. There is a condition know as idiopathic vestibular disease in which all of the symptoms show up acutely, but there is no underlying pathology noted. The good news about the latter condition is that it tends to resolve on it's own in usually a week or so. Here is a link with more information about this condition. LINK HERE (I know the link refers to the disease in dogs, but it really is the same disease we see in cats)
As long as she is still eating and seems otherwise comfortable, you have the benefit of giving her some time to see if she improves on her own as she will if she is affected by idiopathic vestibular disease. You may have to help her by moving her food, water and litter where it is more easily accessible, especially if she has to use stairs to get there. I would not offer her any pain meds that you may have access to as most are not safe for use in cats. If she is not improving in a day or two, I would have you vet evaluate her for further diagnostics.
I hope this is helpful. Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please reply so I can clarify for you.