First, the way you care for your baby extends their life compared to caged. I've seen them live 10, up to 15 years old.
There are a couple things that could make her appear weak in the back and I am going to shoot into the dark and check the obvious, being hock. Sore feet. Check her back feet to see if they are sore.
Now, have you changed the diet lately or introduced any new food? What about crackers? And what is the urine like? Scanty, frequent, incontinent?
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