A 32-year-old multiparous woman, 10 days postpartum, presents with right breast pain, erythema, and induration in the upper outer quadrant. Temperature is 38.8°C. She reports malaise and myalgias. Breastfeeding is extremely painful on the affected side. On examination, the right breast shows localized warmth, erythema, and a tender nodule. Milk expression yields normal-appearing milk. What is the most appropriate next step in management?