On&off left-sided chest pain during exercise today. went away every time i stopped. how do i know if this is muscular related vs cardiac?

Chest pain: For specific diagnosis and advice see a doctor urgently. Here is some info on different causes of chest pain and how they might present for general advice. http://patient.info/health/chest-pain-leaflet

A health care: professional can order an exercise stress test for you and look for characteristic EKG changes or nuclear imaging that will differentiate the nature of your chest pain. It is unlikely that an 18 year old would have coronary artery disease but if you are concerned then make an appointment for evaluation.