My doctor told me if lumps appeared under my armpit during pregnancy (with no history before) It was an indicator of an inability to breastfeed and connected to mastitis. Id get a 2nd opinion to be sure.
I have to say I've not heard of it and personally I would get a 2nd opinion just to be sure that's what it is as unfortunately when you are producing milk or about to or just finishing a lot of doctors will chalk it up to that rather than anything else. Then again I am a worry wort :/ as for feeding with it if that is what it is then you might find feeding reduces it's size. Hope everything works out for you hun x
Every pregnancy causes different problems . Dont get discouraged about breastfeeding . Maybe breastfeeding can cause it to go away since you will be letting out your milk.