Lameness in cats can be caused by a wound, a bite, a damaged claw, a joint problem or another injury to the leg. In older cats osteoarthritis is common and does not always show as an obvious limp. Instead, your cat may stop jumping up to its favourite spot, hesitate on the stairs or sleep more than before.
Watch how your cat gets around and whether it grooms one area more than usual. Do not press hard on a leg that your cat is clearly protecting. The vet often needs to assess movement, joints and muscles and may add X-rays or other diagnostics if needed.
Contact a vet if your cat will not put weight on the leg, has swelling or obvious pain, or if the lameness started after a fall or other trauma. Smaller but lasting changes should also be examined, as cats can adapt and hide pain for a long time.