Any pain in the armpit is a sign to check the area carefully with your fingers. A lump under the armpit is hard and doesn’t move when you touch it. It will feel like a sore or tender spot under the arm. You may not necessarily feel a lump. In some women, the swelling is more prominent under the arm or under the collarbone. The lymph nodes in your armpit are where breast cancer spreads first, by way of lymphatic fluid that drains from the breast. Basically, if a lump or tender spot in the underarm area persists for a week with no apparent cause, see your doctor.