Underarm swelling. Breast swelling is a fairly common sign of inflammatory breast cancer. This swelling may occur around the breast or collarbone but it can also occur in the underarm area. Underarm swelling may be a sign that the cancer has spread to the nearby lymph nodes. Many people diagnosed with breast cancer that has spread to their lymph nodes have had no obvious symptoms or signs in the breast and no changes in the structure of shape of the breast aside from underarm swelling. This swelling is sometimes mistaken for mastitis, or inflammation of the breast tissue usually related to an infection, although mastitis is rare for women who aren't breastfeeding and doesn't occur under the arm.