As other posters have said, she could be pregnant with a very small litter, or she could be having a false pregnancy. You should definitely take her to the vet. He can do an ultrasound in the early stages, if she's producing milk, she's near the end of the gestation and he can do an x ray and tell you for sure. If she's not pregnant, you should have her spayed as she is likely to continue with the false pregnancies time after time. Not good. If she has one or two pups, be very alert to signs of whelping. Take her temp. every day at the same time, when it drops a degree or so, she's within a day or day and a half of whelping. The other dog should NOT be anywhere near while she's having pups, and should not have access to the pups afterward, until they're at least four or five weeks old and only then with supervision. Pups are fragile, especially tiny breeds.
If she only has one or two pups in there, she may not be able to push them out herself. If not, they can easily drown before the vet can help, if you don't recognize the signs of labor going on too long...ask your vet, please. You need to know what's going on with the dog. Or not going on with her!
