I moved your thread to Health & Nutrition, it seems to belong here I think. I am not sure what to tell you, but if your female cat has not been allowed outside where any other male could get to her, and if she is indeed pregnant, then my guess would be that when the vet neutered him, he may have had one of his testicles up inside him and the vet missed it. What I mean is, with my male cat Merlin, I took him in to be neutered and the vet said he thought he only had one testicle, because he had opened him up and looked around for the other one and could not find it, so he sent him home but a few months later, Merlin was spraying and trying to mate one of the females that had not been spayed yet, so I took him back in, the vet opened him back up and looked harder and there it was, he finally found it and now Merlin is really neutered. So this could be a possibility...I would call your vet and ask him if he found both testicles when he was doing this. But some of the other people here may have better advice for you, I'm no expert, just going on personal experience. Keep us posted on what you find out and if she really is pregnant!