A 30-year-old woman on the fifth day after the physiological childbirth complained of swelling of her left breast, pain, reddened skin, and a fever of 38°C. Objectively, her left mammary gland is enlarged, the skin there is red, with cyanosis in the upper-outer quadrant. A sharply painful infiltrate without clear borders can be palpated. Expressing milk does not bring relief. What is the most likely diagnosis in this case?