Cysts (fluid-filled tissue pockets), infections, and swollen lymph nodes commonly cause armpit lumps. Finding a lump in your armpit can be worrisome, especially if it hurts or appears unexpectedly.

It is uncommon for females under age 40 to develop breast ... more likely to happen due to other causes. An armpit lump often occurs due to an infection, cyst, skin problem, or an allergy to ...