Mastitis: This is an inflammation of the breast tissue, which can be infectious or non-infectious. Given the symptoms of pain, redness, and swelling, mastitis is a common and likely diagnosis, especially if the patient has a history of recent trauma, surgery, or infection in the breast.

Breast Abscess: A collection of pus in the breast tissue, often as a complication of mastitis. It presents with similar symptoms and is a consideration, especially if there's a fluctuant mass or significant tenderness.
Fat Necrosis: A condition where the breast fat tissue dies, often due to trauma. It can mimic signs of infection or malignancy, including pain, swelling, and sometimes skin retraction.
Periductal Mastitis: Inflammation of the ducts under the nipple, leading to symptoms like pain, swelling, and sometimes nipple retraction or discharge.

Inflammatory Breast Cancer (IBC): A rare but aggressive form of breast cancer that presents with rapid onset of swelling, redness, and warmth of the breast, often without a distinct lump. It's crucial to consider IBC due to its poor prognosis if not promptly treated.
Necrotizing Fasciitis: A severe infection of the skin and underlying tissue, which can rapidly progress and is life-threatening. While rare, its severity warrants consideration in cases of severe pain, swelling, and redness.

Granulomatous Mastitis: A rare inflammatory condition of the breast characterized by granulomatous inflammation. It can mimic other conditions, including breast cancer, and is often diagnosed after biopsy.
Sarcoidosis of the Breast: A rare condition where granulomatous inflammation affects the breast tissue. It's part of systemic sarcoidosis and can present with a variety of symptoms, including pain and swelling.
Breast Lymphoma: A rare malignancy that can present with swelling, pain, and sometimes systemic symptoms. It's essential to consider in the differential, especially if other diagnoses are ruled out.