I am not that experienced with rats, but I have seen something similar in aging dogs. My older dog is doing this a little with his back legs (although with dogs you can help them out with glucosamine and chondroitin *sp??*...... wonder if that would help rats out??) usually when he is getting up from a long nap or after he's been outside running with the puppy or the kids. But I would think that it is a little too soon to think about putting her down, if she is still able to get around without being in a whole lot of pain and is still doing all of her normal ratty things. I would think that at the moment it is just from her older age.