Lipoma: A benign tumor composed of fat tissue, which is common in adolescents. The fact that the growth is soft and has started to get smaller suggests a lipoma, which can sometimes resolve on its own.

Cyst: A fluid-filled sac that can form under the skin, which is also common in adolescents. The lack of pain, redness, and breathing changes makes a cyst a plausible diagnosis.
Fibroadenoma: A benign breast tumor that can occur in adolescent females. Although it's more common in older adolescents, it's still a possible diagnosis.
Sebaceous cyst: A non-cancerous lump under the skin, which can be soft and mobile. The fact that the growth has started to get smaller could indicate a sebaceous cyst that has begun to resolve.

Breast cancer: Although extremely rare in 14-year-old females, breast cancer must be considered to avoid missing a potentially life-threatening diagnosis.
Lymphoma: A type of cancer that can cause a lump or mass in the chest, which requires prompt evaluation and treatment.
Infectious abscess: A collection of pus that can form under the skin, which can be life-threatening if left untreated. Although the lack of pain and redness makes this less likely, it's still essential to consider.

Dermatofibroma: A rare, benign skin growth that can occur in adolescents.
Hemangioma: A rare, benign tumor composed of blood vessels, which can occur in the skin or under the skin.
Neurofibroma: A rare, benign tumor that arises from nerve tissue, which can occur in the skin or under the skin.