Can my recently castrated dog still get another pregnant?

We have two 6 month old Pomeranians. A boy and girl, the girl came into heat, and the vet wont spay her until afterwards, so we had the boy castrated. It had been two days since castration, and we caught them tied together (obviously mating). Will she definitely not be pregnant? (giving that she has not been with any other male dogs, and this is the first time they've been out of sight together since she started her heat cycle)