If a girl’s breasts do not protrude and seem to have hard lumps inside, it may be due to the formation of breast nuclei, the beginning of breast development, infection of subcutaneous tissue, mastitis and so on. 1. Nucleus formation: Between the ages of 4 and 6, girls will gradually form a nucleus and develop a small hard lump in their breasts. This usually does not last long and there are no obvious symptoms of continuous growth. 2. Beginning of breast development: When the girl’s breasts begin to develop, the breasts do not protrude, but small internal lumps are formed in the breast area, which is a normal physiological phenomenon and parents do not need to worry too much. 3. Subcutaneous tissue infection: Subcutaneous tissue infection is usually caused by bacteria invading the subcutaneous tissue after the skin is broken, usually resulting in localized skin swelling, redness, pain and so on. 4. Mastitis: Mastitis can cause nodules in the breasts and hard lumps in the breasts. Some patients also have fever, skin redness, swelling, pain and other symptoms. The hard lumps in the little girl’s breasts may also be due to breast hyperplasia, breast cancer and other diseases, and it is recommended to go to the regular hospital for examination and treatment in a timely manner.