My male dog was netured a few years ago but mated the other day with a shihzu and now shihzu teets are getting big.Could it be that she is having a false pregnancy or could it be that he truly wasn't fixed?

Married, does he still have either of his testicles? If not, since it has been a few years, he is not capable of impregnating her. You are either dealing with a false pregnancy or is it possible that another male dog got to her? Is this female your dog or is it someone else's and they are accusing your dog of mating with her? I would suggest whoever owns the female make an appointment with their vet and have her evaluated. If she is pregnant and someone is accusing your dog of impregnating theirs, get a written statement from your vet that your dog is neutered and it is an impossibility that he is the culprit.