It's probably a clogged duct, regular nursing and good supportive but not too tight bra helps avoid them. They are awful. Warm shower or warm compress and massage it out even though it hurts. May take a day or two to feel better. If she wont nurse well, you may want to pump after she tries to empty the milk out. Hope that helps. Sorry, i got quite a few after i started working out again after each baby, ouchie.