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How old is your cat?
That is alittle young for true arthritis to develop, but not impossible. I would be concerned there is another issue going on like spinal or disc disease, a neurological issue, or other. I recommend a trip to the vet. We have come a long way with medications for cats...so if it is a back or hip degeneration...at least we can help with pain...if it something else, you can get early intervention.