My guess would be just over-exeration.
Personally, I would not give Rimadyl. I would give an ascriptin or baby aspirin and just rest him.
Disco hurt his toe running in the yard with Dessie a few weeks ago. He was three-legged lame for about 15 minutes and then limped slightly for a day or so. I believe I gave him a baby aspirin shortly after it happened and maybe one later on the afternoon. My guess is he dislocated his toe and it popped back in place with no further injury.
He was on the IR list for about 3 weeks, only going outside with the little dogs. We told him that his job was to protect the little dogs while he was outside with them. He believed it. :lol He is fine now.
:grouphug