Intertrigo: This is a common condition characterized by a red, itchy rash in the skin folds, often due to friction, moisture, and fungal or bacterial overgrowth. The area under the breasts is a typical location for intertrigo due to the warm, moist environment.

Irritant Contact Dermatitis: This condition occurs when the skin comes into contact with an irritant, such as soap, detergent, or fabric softener, leading to a red, itchy rash. The skin under the breasts can be particularly sensitive.
Atopic Dermatitis (Eczema): Although more common in flexural areas, atopic dermatitis can occur anywhere, including under the breasts, especially in individuals with a history of eczema.
Candidiasis: A fungal infection that thrives in warm, moist areas, making the skin under the breasts a potential site, especially in individuals who are diabetic, immunocompromised, or use antibiotics.

Inflammatory Breast Cancer: Although rare, this aggressive form of breast cancer can present with skin changes, including redness and rash, often without a distinct lump. It's crucial to consider this diagnosis, especially if the rash is unilateral, doesn't respond to typical treatments, or is accompanied by other symptoms like swelling or nipple retraction.
Cellulitis: A bacterial infection of the skin and subcutaneous tissues that can present as a red, warm, and tender area. It requires prompt antibiotic treatment to prevent serious complications.

Granulomatous Lobular Mastitis: A rare inflammatory condition of the breast that can cause skin changes, including redness and swelling, often accompanied by a mass or nipple retraction.
Paget's Disease of the Breast: A rare form of breast cancer that can present with eczema-like changes to the skin of the nipple and, sometimes, the surrounding area, including under the breast.