Lactating mother developed Breast pain over her left breast

A 25y/o G1P1, lactating mother developed breast pain over her left breast. PE showed a fluctuant area measuring 4×6 cm around the periareolar area. The overlying skin is erythematous, warm to touch, and tender. Axillary examination also showed a palpable, tender axillary mass. Explain In Detail the anatomical basis of the axillary mass.