Our Jack Russell hurts its leg every once in awhile. We have had it checked and they can't tell anything. It is probably a soft tissue injury that never fully heals because she insist on being a maniac.
Dogs get muscle tears and strains just like we do. You might try to get a second opinion and then if they can find nothing ask what motrin dose would be appropriate. We give our senior dachshund motrin for her aches and pains.