If you don't feel comfortable with your vet's lack of concern, I'd certainly seek another opinion. It must hurt or the dog would be using all four legs.

Scruffy didn't show any signs of pain, but he held his back leg up when he ran. After a few weeks, I realized something had to be wrong. When I took him to the vet, she said he had a luxating patella. Because I waited so long his muscles in his leg had grown very weak. For this and other reasons, he now is three legged. And does very well that way.

I don't mean to scare you. But I wouldn't let it go. SOMETHING is wrong.