Pain in chest and shoulder, won't stop, don't know why?

i was in cl (high school, i'm a teenage girl, no health problems, healthy weight, etc etc) when out of nowhere i started getting this pain in my chest, close to the middle but mostly on my left. this was around four hours ago, and it hasn't stop since then. all it's done is spread to the back of my shoulder, close to my neck, and i feel a little bit sick (but that might just be in reaction to the pain). it's bearable, but definitely not pleasant, and a few times (especially after laughing) i had to curl up and not move until it ped. so yeah i was just wondering what the cause could be?