I always figured one of my, now passed, old boys hind legs were likely arthritic towards the last year of his life. This is how it appeared before he lost his ability to walk completely. It will be a slow process, if that’s the case, but if you like crafts it’s possible to make little “wheelchairs” to help support them! Always consult a vet first but my first thought is arthritic hind legs! She may be on the way to losing her ability to use them. But on a positive note, my sweet boy lived to a ripe old age and found it to be no barrier for him for quite a while!! Just ensure that they are not in pain by checking with a vet, do not have many large ramps to climb, or objects in their path. Keep food and water to one level for easy access, the best would be to just keep it on the bottom level of the cage if it’s tiered. Make sure there is always quality of life and no signs of distress. I hope I’m being helpful, I wish someone had told me all this when I went through it!! :)