Breast pain in 18-year-old girls is considered to be caused by mastocytosis. Mammary gland hyperplasia is a common condition in which the breast cells change periodically with the menstrual cycle, increasing in size before the menstrual period and rejuvenating after the period. If there is an overgrowth of breast cells combined with incomplete rejuvenation, this can lead to mastocytosis. Mastocytosis is a group of symptoms that result from abnormalities in the normal development and degeneration of the breast. Mastocytosis is characterized by breast pain and fibrocystic nodules as the main clinical manifestations. About 70%-80% of women have varying degrees of mastocytosis, most commonly in women around the age of 18-40. Therefore, the essence of mammary gland hyperplasia is abnormal breast development and rejuvenation, showing cyclical changes with the menstrual cycle, not an essential disease state, so do not worry very much.