Luxating patella came to mind.
Sierra has a luxating patella and only bothers her once in a great while (maybe twice a month) and when it does she'll yelp and not put any weight on it then she'll snap her leg backwards (to pop it back in) and she's perfect again!! The next time you visit your vet, for shots and whatever, you should mention it to them, they can move the knee cap around and see if it seems loose to them. It's much more common in small breed dogs, but any dog can have it.