you should have a checkup as soon as possible. it could be several things. heart attacks can have a variety of symptoms from vomiting to pain in shoulders. or pain like yours. other symptoms can mock muscle strain. or acid reflux. don't put your life at stake! run! don"t walk to the nearest emergency room for a workup!
i'm 36 and having these pains on my left side of my chest to the side of my rib cage. doesn't hurt to breathe though. took heart burn meds still there. lots of pressure though. can really feel it when i breath in.