Shih Tzu playing ball on the beach

Our pet Shih Tzu, Oscar with a tennis ball in his mouth on Garrettstown Beach a few weeks before Adam was born last April.
He loves running after the ball, but the poor little doggy is a bit lame right now. We took him to the vet who couldn't find anything wrong. This has happened before and he recovered so fingers crossed he gets better this time too.
The strange thing is, it only affects him when he's walking on pavements or roads. On grass he's almost fine, and at home you wouldn't know it from his running around! Dog psychology?