Mastitis

Does anyone know what causes mastitis four years from original lumpectomy? I'm taking antibiotics for the infection, but there is also a lump that they are going to biopsy. This has been very confusing to me because when I read about it in Dr. Love's breast book, it relates more to nursing mothers. Would appreciate your comments on this one. I'm scared of the results of the biopsy, but the pain from the infection is horrible. Just want to make sure I'm asking for what needs to be done. THanks for your help.