I will assume you are speaking of a cyst. Many things can cause the formation of a cyst. They can be infected or not infected. A good example of a cyst is in the female who shaves her underarms or axillary area. Sometimes when women shave this area a small ingrown hair will be the initial site of the cyst. The tissue is irritated by the hair and the body responds by inflammatory processes which causes the area to swell. Pain in the area may be the first symptom, it may also be red, tender to the touch, and heat may be felt in the area. Some of these will be infected, others will not. With a cyst in this area of the female it is important to determine exactly what the growth is because some forms of breast cancer can spread to this same area and may be felt as a cyst.
I would encourage you to go see your primary care physician to determine the etiology of this growth.