It is extremely rare for girls her age to develop breast cancer, and this occurrence are most commonly due to benign conditions such as those related to hormonal changes during puberty. Most of these lesions would regress spontaneously.
This can also be due to a localized form of infection, which can be treated with a course of antibiotics.
To allay your fears further, it is best to have this evaluated personally by her doctor. Regards and God bless.