My vet once told me that dogs do that if their hips are either really good or really bad. Just for grins and giggles someday when she is mature, you may wish to have her hips x-raayed, just to feel better.
My Aussie, Striker, did it all the time. His hips OFA'd as good. I also had a kitten who argued with a truck, lost, and broke her pelvis. Once it healed, she slept like that. Really weird to see a cat do that.
I've had all breeds of dogs from chihuahua to lab and mixed, even the baby puppies would do it, either lay like that or pull them self along the ground, the preg. females would do it as if scratching their bellies, then get up smile and shake.