This is not meant as a criticism of your vet, so please take it at face value: Even the best professional can't be right 100% of the time. Ask yourself if this is a conclusion that makes you comfortable. If this were your pediatrician telling you this, would you be satisfied, or would you seek a second opinion, either from another pediatrician or from a physician with more expertise in the area? Sometimes, when a problem with our pets does not abate or disappear with standard veterinary practices, your vet can refer you to a veterinary specialist. It's no reflection on your vet's capabilities, as you must remember, veterinary medicine is infinitely more complex than human practice. Vets have to have knowledge of different species. A vet's patients can't describe what the symptoms are.
Did you know the requirements for admission to veterinary school are more stringent and it's more difficult to get into a veterinary college than it is to med school?
This just occurred to me. Has her physical activity and exercise decreased? Maybe there is a loss of muscle tone that is allowing these muscles to be overly relaxed when she sleeps.