Mastitis: This is an inflammation of the breast tissue, which can cause sudden onset breast pain. It is often associated with lactation but can occur in non-lactating women as well. The pain is usually accompanied by redness, swelling, and warmth of the breast.

Breast Cyst: A breast cyst is a fluid-filled sac within the breast. It can cause sudden onset breast pain, especially if it becomes large or ruptures. The pain is often cyclic, varying with the menstrual cycle.
Fibrocystic Changes: These are benign changes in the breast tissue that can cause breast pain. The pain is often cyclic and can be accompanied by lumps or nodules in the breast.
Trauma: Direct trauma to the breast can cause sudden onset breast pain. The pain is usually localized to the area of trauma and can be accompanied by bruising or swelling.

Breast Cancer: Although less common, breast cancer can cause sudden onset breast pain. It is essential to consider this diagnosis, especially if there is a palpable mass or other suspicious findings.
Pulmonary Embolism: In rare cases, a pulmonary embolism can cause referred pain to the breast. This is a life-threatening condition that requires immediate attention.
Cardiac Ischemia: Cardiac ischemia or myocardial infarction can cause referred pain to the breast. This is a medical emergency that requires prompt evaluation and treatment.

Pectoralis Muscle Strain: A strain of the pectoralis muscle can cause breast pain. This is often related to trauma or overuse.
Tietze's Syndrome: This is a rare condition characterized by inflammation of the cartilages that connect the ribs to the breastbone. It can cause breast pain and swelling.
Sickle Cell Disease: In patients with sickle cell disease, a sickle cell crisis can cause breast pain due to vaso-occlusive episodes.