Hi - This doesn't sound like a breast cancer symptom; breast cancer itself is seldom accompanied by pain. Could it be an infection of some sort, considering your underarm gets swollen and lymph nodes seem to be involved? If your doctor doesn't offer you any more information than "it's from nursing," I'd press him or her on it to find out exactly what it might be. If you don't get satisfaction - get another opinion. It's good to advocate for yourself, even though it takes time/energy. So pursue it, but don't stress, OK? Good luck - PJH