I'd need more info. is this an older cat? It could be feline arthritis. If a 6-7 year old (roughly) it could be early onset arthritis due to other circumstances (osteo issue). It could be a constipation issue as mentioned above.
does your cat seem to be in pain? to touch? Does it seem more back or hind leg related? Is this a male or female? how is his stool/urination?
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