Well I had an orthopedic specialist tell me he could not rule out a disc issue after a clear x-ray without doing a myelogram (or an MRI)
Sudden onset does not sound like DM, neither does the young age and one of the first things people normally notice is the toe issue with them scraping their back feet.
There are any number of injuries that may present without pain. I would think crate rest, and some oral steroids would be first on the list. Tick disease can also cause this.
My Brandy, GSD from many years ago, diagnosed with DM before I even heard such a think existed, was 9 when he first started showing symptoms, and it came on very gradually. I do remember also that his hindquarters started to sway slightly when he walked, and the muscletone in his whole back end slowly left. This was all before the symptoms were obvious to the casual eye.