Lumps under the armpit assciated with the immediate postpartum period could be
breast tissue in the armpit.  Occasionally, breast tissue can appear in
auxilliary breasts anywhere along the nipple line from under the armpit
through where the usual nipples are to a line caudal to the nipple (imagine
the nipple line that dogs have and you get the general idea).  If this is
aberrant breast tissue, I doubt it needs much treatment.  Hopefully, it will
involute if it isn't drained by nursing.  Maybe some of the OB/Gyns on the
list have seen this before.
Linda L. Shaw MD,  Pediatrics