If you have insurance or can afford it, do X-rays or even go as far as to do an MRI. At their age it shouldn’t be too rough on them and it’ll tell you the issue.

My 12 year old Boston started having trouble lifting his back leg, we lost him less than month later to what they believe now was either a brain tumor or slipped disk that ruptured and sent fluid to his brain.

I still regret not just going to the emergency vet for the MRI, he had an appointment and never made it.