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It sounds like your cat is having a back or disc problem. You really need to see the vet to have it checked out. He might need steroids and/or anti-inflammatories to help deal with any pain.
This is not common in a cat this age.
Let me know if you still have questions.
Over the counter medications are not safe for cats so I cannot suggest one.
A vet can prescribe something that would work well, if that is an option for you.