The majority of dogs are born w/ round worms. Passed by their mothers placenta (I think, I know it passed through the mom somehow even though the mom may not have worms the pups can still get them easily before born).
Most pups are de-wormed from the breeder & also by the vet when you go in for puppy vaccines.
Are you sure your vet did not do this when she was a pup? Even if not & you had her stool checked recently & it is clear I would not worry about it.