Some dogs do that, and there doesnt seem to be a problem that is making them do it. They are just keeping themselves clean, and perhaps leaving a scent trail also as I understand. However (it's unlikely at 6 mnths, but later in life) if your dog is doing it very often and starts to show discomfort as she sits down it could be that she has an absess/her anal glands need cleaning out. In some dogs these glands become blocked, and need squeezing by the vet to clear them.
sportsmonki