Breast bud tenderness or fibrocystic changes: This is a common cause of breast pain in adolescent females, especially during the early stages of breast development. The pain is often cyclic and can be associated with the onset of puberty.

Hormonal fluctuations: As the patient is 14, hormonal changes associated with puberty can lead to breast tenderness.
Costochondritis: Inflammation of the cartilage that connects the ribs to the breastbone can cause breast pain, especially if the pain is localized to the chest wall.
Musculoskeletal strain: Strain or injury to the muscles of the chest wall can cause unilateral breast pain.
Infection or inflammation of the breast tissue: Although less common in this age group, an infection or inflammation of the breast tissue (e.g., mastitis or periductal mastitis) could cause unilateral breast pain.

Breast cancer: Although extremely rare in adolescents, breast cancer must be considered, especially if there's a family history or other concerning features.
Pregnancy-related conditions: Despite the patient reporting not being sexually active, pregnancy should always be considered in the differential diagnosis for breast pain in females of childbearing age, as it can be a cause of unilateral breast pain.
Tuberculosis of the breast: This is a rare condition but can cause unilateral breast pain and should be considered, especially in areas where tuberculosis is common.

Breast cysts or fibroadenomas: While these are common benign breast tumors, they are less likely to cause significant pain unless they become large or rupture.
Pectus excavatum or other chest wall deformities: These conditions can cause musculoskeletal strain leading to breast pain but are less common causes.
Neurological causes: Conditions like thoracic outlet syndrome or nerve entrapment can cause breast pain but are rare in this age group.