Even if she is pregnant, you can still spay her if she's still in the early stages. Dogs can indeed have false pregnancies and it isn't uncommon. To be on the safe side, though, I really recommend you spay her now. Many vet clinics can fit you in by the end of the week, so there is absolutely no reason not to spay her immediately.

ETA: And when a dog is neutered, the testicles are removed completely and the tubes connecting to the former testicles are tied so the chance of the dog procreating is almost non-existant.