I hope you have read the comment I made to Dakota's mom on Dec 5th because your question has a very similar answer. Your dog can have anything from Lymes disease, to a soft tissue injury(sprain or strain), mild or severe traumatic injury such as a broken nail, torn right anterior cruciate ligament, spinal injury and many, many other possibilities best sorted out by your vet. Due to dog's accelerated life span, damage to joints by injuries are also accelerated, for example a knee injury in a dog if untreated may result in arthritis within a couple of months after an injury. The same injury in a human may result in arthritis many months to years after the injury.
Please tell me you've taken your dog to the vet by now?
To get a better idea of what is going on, you'll simply need to put a leash on your dog and don't allow him to walk away while you examine him.