I have chest pain, but it only hurts when I stretch. It also is located in the upper left side of my chest. I'm only a teen.?

Musculoskeletal pain. It is very unlikely to represent serious issues, such as a cardiovascular origin. If it is reproducible with certain movement, it is almost certainly related to muscle strain from exercise or past injury. Unless it is severe, it is probably worth giving some time and possibly a short course of anti-inflammatory medications.
Muscle strain. Most likely is a muscle strain, but an exam by your health provider is the only way to know for sure.