A lump in the armpit is almost always a swollen lymph node. Lymph nodes can swell for many reasons including illness, infection and other hormonal changes. However, since the armpit is quite close to the breast so treat an armpit lump the same as you would a lump in your breast. If you're pre-menopausal, wait a menstrual cycle and see if it goes away. If your post-menopausal see a doctor right away.