Agree with ginger. Sounds like infection and she needs to be seen by a Vet.
Is she nursing puppies at the moment? If so this could be a blocked milk duct plus it sounds like some kind of infection going on in there, maybe even an abscess. I think she needs to see a vet urgently in that case. Apart from the pain and discomfort, this could seriously affect the puppies.
If she's not nursing right now, then it could be a breast cyst. And the best thing for that is a needle-biopsy, to see what this lump is.. If the lump is infected -antibiotics as well