I would recommend going to see your primary care doctor
- such as your pediatrician
or your adolescent medicine doctor - about this issue. In a young woman like yourself, breast discomfort is very rarely a serious medical problem, but it definitely is worth having it checked out by your doctor.
One of the first things that will be helpful when you go to see your doctor is if you can think back to whether or not the breast pain is associated with your periods. Breast tenderness or fullness, especially shortly before the period, is a common symptom which many women experience. If so, this may be part of a "premenstrual syndrome" which may not require any treatment if it is mild, but may respond to exercise, dietary changes, or over the counter pain medications as indicated by your doctor.
Another common cause of breast discomfort in young women is tightly fitting underwear, or poorly supported breasts during physical activity. Both of these problems will usually improve with a properly fitting bra or sports bra.
Rarely, breast pain may be from a more serious breast problem, such as a lump or a mass in the breast, and your doctor will be able to perform a breast examination to rule out these problems.