At your age, the most common reason for this is still hormonal changes. For some women, the breasts tissues react differently to hormones causing intensified pain that can persist throughout the menstrual cycle. Other possible causes would include musculoskeletal strain which can be due to stress. However, I would suggest that you have your breasts examined professionally by your physician, and possibly have a mammogram if your doctor deems it necessary.
Regards and God bless.