under breast

http://hidradenitispatients.com/hidradenitis-suppurativa-natural-treatment/ -

This frequently occurs as a consequence of a hair follicle or an oil -producing sebaceous gland inside the follicle becoming damaged [source: Mayo Clinic]. On your own face, for example, such damage might be caused by acne. But any kind of damage to the skin, including a wound or exposure to ultraviolet rays that are dangerous, can create the conditions that form a cyst.