Discovering a lump can be concerning, but many red lumps under the nipple are benign. Potential causes include a skin infection, a clogged sweat gland, a benign cyst, or a localized skin condition, though breast cancer must be ruled out. To determine the exact cause, a healthcare provider will perform a physical exam and likely recommend imaging tests, such as a breast ultrasound or a diagnostic mammogram. A biopsy may be performed if the imaging is inconclusive. Medications (such as antibiotics for an infection) are only prescribed once a definitive diagnosis is made. Please schedule an appointment with a doctor promptly for a proper evaluation.