I'm not sure what to tell you. If the vet says she's OK, then she probably is.
I have multiple dogs and 2 Yorkies. They have very intensive periods of play (and fetch!), then long naps. Then more play (and naps) all day up until bedtime, when they go to bed for a l-o-n-g nap
You said you just got her. Maybe her former owner was more active at night than during the day, so she thinks "day" is "night"?
Hope you feel better soon ...