Tingling in the breast when lying on the side may be caused by improper sleeping position, trauma, breast hyperplasia, mastitis and other factors. 1. Improper sleeping position: If you sleep on your side, due to the improper sleeping position, the breast will be locally compressed, affecting blood circulation, which may lead to tingling when lying on the side of the breast. 2. Trauma: If the breast is hit or violent, it will easily lead to soft tissue contusion, congestion, edema, etc., causing breast pain when lying on the side. 3. Breast Hyperplasia: If the patient suffers from breast hyperplasia, the patient may be affected by the level of estrogen and progesterone in the body, causing local glandular hyperplasia, which will cause tingling when lying on the side of the breast. 4. Mastitis: When a woman suffers from mastitis, if there is an acute suppurative infection of the mammary gland, under the stimulation of inflammation, it will easily lead to redness, swelling and pain, and the phenomenon of tingling when lying on the side of the breast. In addition to breast cancer, breast fibroadenoma may also appear similar symptoms, the patient appeared breast side lying down pain should go to the hospital breast surgery in a timely manner, clear the cause of the disease and follow the doctor’s instructions for treatment.