I would get your dog in to the vet and have him/her gently manipulate your dog's back legs to see if there is a range of motion problem. If there is pain, then you may want to get x-rays to find out what is going on. I'm not sure if a five month old pup is really grown up enough that an x-ray would show a definitive answer though. That's still a very young pup.
I don't think your pup is too young to start on a joint supplement though (check with your vet first). It can't hurt.
Niko: American Showline GSD 5 years old
Rosa: American Muppet Dog (GSD/Border Collie mix) 5 years old