You have already listed most of the possibilities. A small amount of breast tissue normally exists in the armpit area (and more so in some people than others). Therefore, any breast abnormality can manifest in the armpit area. The fact that this lump is more prominent around the time of your period suggests (but does not prove) that you may have some fibrocystic changes in the "tail" of the breast. (Fortunately, breast cancer is extremely rare in people your age, but it cannot be completely excluded based upon age alone.)
There are also multiple lymph nodes in the armpit area, and these can become enlarged as well. In most cases, enlarged lymph nodes are caused by benign inflammatory of infectious conditions. Rarely, however, lymph nodes can enlarge due to lymphoma (a cancer of the lymph nodes), or due to the spread of cancers from other areas of the body.
You should make an appointment to see your doctor around the time of your period, for an examination. If a lump in your armpit persists, then it might also be helpful to have an ultrasound of this area performed as an initial diagnostic step.
Sincerely/ Robert A. Wascher, MD, FACS
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