I suggest popping over to your PCP or, like nikkie said, a low income or free clinic, if you have any in your area. It might just be a pulled muscle or something innocuous, but it could also be serious. Do you have any bruising or redness in the area the pain is in?
I know medical bills are expensive, but if you have a family history of heart issues, it's better safe than sorry; ESPECIALLY if you've done any damage. Most hospitals are willing to work with you on payment plans for your bills. I know it sucks, but if you've fractured a rib or seriously screwed up a muscle in your chest, it's better know about it and get a diagnosis than to be in the dark and risk doing more harm, making it even more expensive down the road.