I do agree that she most likley is not pregnant, but however since you did know for sure she was in heat and that her body had changes in the past 63 days, I would have to aggree that bringing her in for an ultrasound or even have the vet palpate her( which is less expensive). My vet only charges 50.00 for a Ultrasound.I would also press on her nipples to see if any milk does come out.
With inexperience dogs doing the tango together sometimes it does not produce pups. Which is a good things as she herself is just a baby.
Over the years I have had a few of my labradors go thru false heat and false pregnacies which is never fun. Best to ease your mind and have her checked.