Shar pei back legs problem?

Ive got a 5 month old shar pei and for the past 2 months or so her back legs keep popping out of place gets quite bad sometimes, was at the vet when it started and they couldnt find anything the matter with her legs which is just weird as its quite bad at times like a walk that usally takes 5 mins will take 20 mins as the dog has to keep sitting down as she can walk