With any chest pain/discomfort, you should see a physician ASAP. Also, take aspirin. Just because you have pain in the left part of your chest, it does not mean you have any heart-related illness....
PMS if you're a girl. It's usually on the left side. If not that, it could be gas pains or maybe just growing pains. How you slept, what you eat, things like that. OR It could be UTI (Urinary Tract...