Adrenaline is an amazingly effective short term pain relief - I have seen Sophy go from unable to move from back pain to dashing to the door at 60 mph. It doesn't mean the pain or injury have gone away, however, and the dashing may well make it worse once the excitement has worn off. I would want to investigate further for a pattern - is it almost always on the way home? At the same spot? After both long and short walks? Was she immediately bouncier on metacam (nearly always a sign of pain without it)? If it continues, and your vet can find no explanation, I would be tempted to ask for a referral to a fully qualified animal osteopath - I found one who made a huge difference for Sophy's pain and my anxiety!