It could very well be pano (growing pains). My puppy is almost 7 months and started experiencing the same thing. I've had him on adult food with a lower protein since 16 weeks and he did fantastic. I recently switched him to a high performance dog food and he started limping. The protein level in his old food was 26 %; when I switched his food it increased to 30%. I just switched him back. Still going to the vet tomorrow just to rule anything else out. Check your puppy's food for the protein level...hopefully that is it. It temporary, but painful for them. My guy also had a decreased appetite. I have him in the crate for some rest so he doesn't injure himself hobbling around. Good luck!
Isen Vom Kavallerie - DDR/Czech GSD (pup in training)
Red - Rescue GSD (CGC)
Gypsy - Yellow Lab (BH, CGC)
At the Bridge....Charlie - GSD (2014) and Shiloh - Lab (2010)