Maybe an injury during playing? I remember a pup of mine once, around 6 kilos, tried to get infront of an AmBulldog, 50+kilos, to cross a door while palying with each other. As a result he injured his front knee and it showed for some time. The greatest concern was the case of hurting the cartilage/ growth area since it could cause lack of development, bones grow from each end, not all over.