The most common cause is folliculitis, which is caused by bacterial infection of the local skin and usually occurs after trauma. The main manifestation is a localized red papule or several papules, which are hard and have a smooth surface, with obvious burning and painful sensations, and the pain is significantly aggravated when pressed, and usually subsides on its own in 1-2 weeks. The bumps are flesh-colored and do not hurt or itch. Consider an epidermal cyst, this is caused by high sebum secretion and poor drainage, the surface of the bumps is smooth and hard, sometimes pushing the bumps can move under the skin, usually without any conscious symptoms. Some pregnant women may have scattered hard bumps on the areola, which are Montgomery’s nodules, caused by the high activity of sebaceous glands during pregnancy. Some newborns, especially female infants, have a small bump on the nipple or areola, which is not painful or itchy and sometimes increases slightly when heated, due to the high estrogen level in the body.