If she's already been mated, I would ask a vet to check her in any case. When was she mated? What kind of dog is she? How old is she? Has she only had one pregnancy before? Is she a pedigreed dog? If so, did her mother have any problems? You should be able to contact her breeder if this is a pedigreed dog.

But most of all, of course, take her to the vet to get her checked out.