The causes of breast pain after lying down include costochondritis, mastitis, breast hyperplasia and trauma. 1. Costal chondritis: Inflammation occurs when the cartilage at the junction of the sternum and ribs is infected by bacteria or fungi, and the inflammatory factor stimulates the surrounding nerves, thus triggering pain. 2. Mastitis: Infection of the mammary gland by bacteria or fungi leads to damage of the mammary gland, and breast pain can occur after lying down and getting up. 3. Mammary gland hyperplasia: the overgrowth of breast tissue is caused by various factors, and the pressure of the hyperplasia on the nerves will cause pain and discomfort in the breasts. 4. Trauma: when the breast is damaged by external force, the symptom of breast pain after lying down will also occur. Finally, if you have breast pain after lying down, please consult a doctor in time.