If the lump is painful,then could be a clogged milk duct which is very common in
breastfeeding mothers.Your should massage the lump
while breastfeeding or when pumping.Applying very warm compresses should help unclog the duct.
However,any new lump that persists,whether painful or not, needs to be evaluated.Since you are breastfeeding,it's highly unlikely to be something alarming ,but the only way to get a proper evaluation is to have this lump investigated by your OBGYN.