Not quite the same situations, but salem started that, he would walk on 4, run on 4, but when he trotted ( horse term i don't know what dogs do LOL ) he used 3. I took him to the vet and the doctor found a bit of arthritis in that knee. since he was 2 or 3 at the time i was like huh? but turned out he had lyme disease. which apparently gets checked by the same type as heart worm scared me to death when they walked in with it and said its positive. but anyway the arthriitis was caused fromt eh lyme disease. not saying its lyme disease,, but maybe arthritis.
although i just looked and your pup is only 5 months, probably not that. but a good thing to know anyway so i'll post it anyway LOL. My mutts both had lyme disease because i didn't know the signs ( they were both on frontline, but i was in the capitol of tick country in the US, so )