A 32-year-old multiparous woman at 3 weeks postpartum reports sudden onset of severe unilateral breast pain, swelling, and a palpable lump in the right breast. She has been exclusively breastfeeding. Vital signs are normal; there is no fever. On examination, the lump is firm, mobile, and non-tender to deep palpation. What is the most appropriate investigation to rule out a serious underlying pathology?