There's a pain on my left side of my chest next to my breast. which it feels like sharp pain but not too painfully. which it doesn't happen often. ?

Time for check up: Anything near there can cause pain: heart, lungs, muscles, breast. This requires a doctor visit.

Probably benign.: Chest pain has a large differential diagnosis. In someone this young, it is most likely related to exercise /muscle overuse. There are some very rare conditions in young people that warrant cardiological evaluation. If pain gets worse or stays long, see your doctor right away.