Has my dog had babies?

I rescued a female Neapolitan mastiff 1 and a half years ago. All we were told was that she was about 9 months old. I think she's had babies before, because her are very large. She's had her period about 3 times since we've had her. Every time I take her to the vet, I never ask, and they have never brought it up. I noticed today that she's lactating out of 2 . She's not pregnant, cause she's not around any male dogs.