A 27-year-old woman who is 7 weeks postpartum woman presents with right breast pain and swelling. She is breastfeeding her infant exclusively without any difficulty. She first noticed redness on her right breast a week ago, which has worsened. She took ibuprofen and applied ice packs with little relief from the pain. She has a temperature of 99.5 F (37.5 C) and is hemodynamically stable. Examination of her breast shows a patch of erythema extending from the areola to the lateral edge of the breast, with a 2.5 cm area of fluctuance at the lateral border at the 4 o'clock position. What is the most appropriate next diagnostic step?