Even though the lumps in the axillary (armpit) area developed during
pregnancy, the woman might consider other possible causes unrelated to
pregnancy and lactation.  Example:  my 15-year-old daughter had several
lumps (enlarged lymph nodes) in both armpits, at different times, which
she said were painful to palpation.  I was concerned, and was
considering taking her for ultrasound, mammogram, etc.  However, I
suggested first that she switch deodorant/antiperspirant products.
After she did so, the lumps went away (slowly) and she has had no
further problems.  Consider soaps, detergents, razors (if used), other
environmental exposure, or even a low-grade, sub-clinical infection
along with other possible causes.  I have worked with a mother who
developed breast cancer while nursing, but that seems to be an extremely
rare situation.  Still, for the mother's peace of mind, she would
probably benefit from finding an answer, if possible.

M. Streiff, RN, BSN, IBCLC