Oh, that poor sweet baby! I am so glad you rescued her!
Mammary tumors are very common in puppymill dogs. The poor things are bred to death and don't get the proper medical care. Unspayed females are at a much higher risk for mammary tumors anyway.
The only way to know for sure what it is is to take her to your vet and have the lumps biopsied.
I'm sure this poor thing has other health issues that need attention, too. It breaks my heart to think of these "invisible" victims of pet store puppies. People don't realize that by buying a puppy from a pet store you are condemning these poor souls to miserable lives in puppy mills to produce those puppies.
Please keep us updated on her. BTW, what did you name her?