Puppy's head is wobbling like a bobble head doll. Has anyone seen this before?

I have a 4 mo. old Pug/ Boston Terrier mix. The other day his head began to wobble back and forth at an even pace. He would also cry out in pain at random moments, and then be fine. It is not a fast shake, like in head tremors. It happens when he is sitting or standing. It never goes away. He was lethargic for one day, but has otherwise returned to his normal self, except for the wobbling head. His appetite is normal, he doesn't drink excessively, he plays with my other dog, urinates and deficates normally. Everything is normal, but the head wobble. I took him to the vet and she couldn't diagnos it. He did have low sodium, but his other electrolytes, including Potium were normal. I was just wondering if anyone else has seen this before? I cannot find anything on the internet. It's all about head tremors, which this is definitely NOT. I have been giving him Pedialyte to bring up his sodium and also sprinkling a little sodium on his food. I'm just perplexed. Need more information.