My puppy was limping a week ago, but had no obvious signs of where pain was when he examined the leg. Took him in to vet, and same thing, no sign of trauma and they could not pinpoint the pain. He was put on 10 days fo rimadyl and rest (no long walks or dog parks), and they diagnosed him with having soft tissue damage (or a bruise basically). After 8 days he now has stopped limping completely and seems to be doing much better. Hope it's the same for your dog, and he's just got a bit of a bruise that needs rest to heal.