A hard lump under one nipple of a girl may be a normal development phenomenon, or it may be caused by trauma resulting in hematoma or breast fibroma. 1. normal development: girls have hard lumps under one side of the nipple, which indicates that the mammary glands have developed, you can ultrasound to determine whether or not the nucleus of the breasts, women usually around 11 years old will have breast development, which is a normal sexual development. 2. hematoma caused by trauma: local hematoma caused by trauma or impact to the chest, when the hematoma disappears, the hard lump will disappear. 3. Breast fibroma: usually caused by hormone level disorder in the body, elevated estrogen level, resulting in glandular fiber proliferation, manifested as painless hard lumps. At the same time, girls with hard lumps under one nipple should also consider mastitis, breast hyperplasia and breast cancer. There are many reasons for girls to have lumps under one nipple, so it is recommended to consult a regular hospital for a clear diagnosis and standardized treatment under the guidance of a doctor.