mammary gland mass

My 11 year old female long-hair, Sooty, has a small lump on her lower stomach.  Vet said it is a "mammary gland mass," and she thought it is probably a tumor, cancer.  Sooty isn't eating well and has a low white blood cell count.  The vet said let's get her over that and then bring her back in about two weeks about the mass.  She gave Sooty an antibiotic shot.  Does this make sense?  Does anyone have any personal information about mammary gland masses/tumors?  What will most likely be the treatment options, as well as prognoses?  Thanks for any info you can give me.

The shot he gave her was Convenia and is used to treat infections.  Her white blood cell count is low because she's got an infection.  It could be from the mass or it could be from something else.  Personally, with the mass, I'd be getting a second opinion.

Thanks.  My regular vet is on vacation and we saw a vet who was standing in for him.  Do you think it is likely that it is not a mammary gland mass?