Left breast distention and pain may be associated with left pleurisy, or acute mastitis or breast hyperplasia. 1. Left pleurisy. If you have left pleurisy, it will lead to fluid accumulation in the left chest cavity, and during the attack, it is easy to cause swelling and pain in the left breast. 2. Acute mastitis. Women during lactation are prone to acute mastitis. If they often feed with the left breast, they are prone to breast siltation, which will lead to mastitis and pain in the left breast. 3. Hyperplasia of breast. If the hormone secretion in the patient's body is disordered, it is easy to cause hyperplasia of the breast, which will lead to lumps in the breast with pain.