being a breast surgeon and onco surgeon, i would little disagree to dr jagdish, yes apparently it could be infected galactocele
, as you are feeding mother.
generally the small bags of milk presenting during lactation
, has a duct draining into nipple. this duct gets block developing a small bag of unreleased milk and later infection sets in, developing infection and fever
but i would agree with dr jagdish for the treatment, but i would also suggest you to get it examined by a surgeon