There are many different things that could be causing the signs you are describing. However, the first thing I think of would be disk disease. The first sign we see with a disk problem is pain, and it can progress into weakness and a drunken walk.
I would recommend having your little girl examined by the vet. If it seems likely that she has a disk problem in her back, you vet will likely recommend a pain reliever/antiinflammatory and an exercise restriction. These guys should be kept quiet and not allowed to run, jump, or do stairs for a few weeks at least.
I hope that helps.
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