The diagnosis depends on what comes out of the lump.  If it is pus,
then it is an abscess.  If it is milk, or changed milk, it is a
galactocoele.  If it is something else, it is something else, usually
a cyst.  The presence or absence of fever, or other symptoms is
irrelevant.  Though fever is a feature of mastitis, it is not a
feature of breast abscess, though it could be present, depending on
how much inflammation there was in the surrounding tissues.

Jack Newman, MD, FRCPC