This lump may be fibroadenomas because these are more frequently in adolescents.Do not change in size with menstrual cycle and do not produce pain or tenderness.
You should do an ultrasound
to differentiate from the cystic mass because in this age mammography
is rarely indicated.
In 30% these disappear,12% become smaller.
Is considered conservative management,you should follow-up every 6 month
Take care of your self