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 personally think it's cute.
Your pup is adorable, too.