I have a toy poodle, Max . When he runs he holds up his back left foot and sometimes his right, sometimes it looks like he is skipping. I have had it looked at by his vet and a bone doctor. They can not find anything wrong. He is 3 years old and weighs 6 lbs.
When a toy breed dog limps on one or both hind legs the diagnosis is luxating patella until proven otherwise.
The patella is also known as the knee cap. Small dogs are prone to patellas that luxate, or slide out of their normal groove. When this happens the dog will limp, often with a hopping or skipping affectation, on the affected leg.
Dogs that suffer chronic pain from luxating patellas sometimes require surgical correction of the problem. Dogs that only limp on rare occasions usually no treatment or intermittent anti-inflammatory pain killers.
I’d recommend that you have another vet take a look at your dog. Ask him or her to assess the patellas specifically. I’ll bet the vet will discover they’re loose.