Mary, I had a chi/dash mix little dog that used to do that all the time. I think when they are so tiny they just don't stop right and the back end keeps going. I doubt that it will cause any worry for you, and as she grows she may stop doing it. It is no more stressful for them than standing on their hind legs and walking on them for a few steps.
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We just bought a baby yorkie, 8 weeks old. This is the first yorkie I have owned. She does something that is real cute but was wondering if it is common? When she junps over a toy or something her hind legs come off the groung and she walks around with them up in the air for a few seconds. She walks around find and her first vet visit was good. Is this something to worry about?