Sudden onset of pain in the left breast needs to consider whether it is a breast problem or a heart problem, as follows: 1. Breast problems: such as mastitis or trauma and other factors, can cause sudden pain. Mastitis by breastfeeding is not timely or due to the lack of conditions for breast pumping, resulting in excessive milk stagnation in the breast, nipple cleaning is not timely, residual milk on it will breed a large number of bacteria, bacteria will be retrograde upward infection of the mammary gland will also occur inflammation. Chest is mainly composed of breast tissue, when subjected to external violent impact or other causes of trauma, will lead to soft tissue contusion; 2, heart: such as acute myocardial infarction, acute angina pectoris or myocardial ischemia, you should urgently need to consult a doctor, do a good job of the relevant examination to avoid delay.