There can be several causes for pain and swelling under the arm and in the breast. One common cause would be a swollen lymph node in the armpit. This can occur for several reasons including a scratch or infection in the arm. Sometimes, swelling of the lymph node can lead to formation of an abscess
or collection of pus. Collections of pus can also form after infection of a hair shaft or a pimple forms in the armpit. Pain in the armpit can easily radiate down into the breast. If you have a painful, red, swollen area in the armpit it should therefore be investigated as many of these require lancing of the pus or taking oral antibiotics to get rid of the infection. Other cause of pain and swelling in the breast can include an infection of the breast, called mastitis. This is unusual in women who are not breastfeeding, but it can happen. If you have a red or inflamed area in your breast, this should be checked out by a doctor
as well for potential antibiotic treatment. Rarely, swelling under the arm can be a sign of a more serious illness, such as a form of cancer called lymphoma. Regardless, the best first step is to have a good physical exam by your primary care doctor
as soon as possible.