There are so many reasons why  lactating breasts can have
lumps. Why not have a needle  aspiration (My medical  knowledge
is limited but at a local conference a  surgeon talked in depth
about needle  aspirations.)  If it  is cancerous a granular substance
is obtained which requires further investigation.  More than likely
a milky substance would be obtained  or perhaps it could be an
abscess in which case greenish black  fluid is obtained.
Ultrasound can help in some cases either to disperse the
lump or to perhaps show if it is a galactocele.
The Breastfeeding Answer Book has some useful information
( p426-427 )  about  breast lumps.
Ruth Bacon  IBCLC  Ontario