Mastitis is an inflammation of breast tissue that is usually caused by a blocked milk duct or bacterial infection entering through cracked or damaged nipples. It causes breast pain, swelling, warmth and redness, and sometimes fever. Risk factors include previous mastitis, sore or cracked nipples, tight bras, and stress. Treatment involves antibiotics, pain relievers, continuing breastfeeding, and lifestyle changes like proper latching and breastfeeding techniques.