Not sure if this is the same thing but both my daughters had what the CHN said was mastitis in both breasts. They both had a hard lump under each nipple. My youngest didn't clear up till she was 4 months. The CHN said it is caused by the mothers hormones passing through the womb and then through the breast milk.
If you suspect it isn't this perhaps you could tell your doctor that you want to know what it is and demand that it be looked into. Personally I feel that any abnormal lump needs an adequate explanation as it is possible that it could be something to be concerned about.
4 lovely munchkins DD 10/03, DD 03/05, DS 10/06 &