This is old but just incase someone searched and found this. When I was 15 I use to get pains in my heart extremely bad, never knew when they were going to hit. My instinctive reaction was to hit my chest over my heart with my fist, and it would usually subside. I would also get pains like someone stabbing me in the heart. I told my parents about this and they insisted on taking me to a heart dr. I ran through all the tests, believe me theirs a lot. Also, they had me wear a heart monitor for several weeks and I was instructed to press play when this was happening with my heart. Well just my luck these pains that were happening all the time, didn't happen once when I had the monitor on. They told me my heart was healthy as an OX. The doc explained to me how it was the muscles around my heart that was causing the pain, then that led to anxiety because I thought I was having a heart attack, which led to panic and my brain would fool me into having more pain. In conclusion, this is usually, I'm not a Dr so don't quote me on this, some sort of growing pain which causes anxiety then gets out of hand. If this happens just focus on breathing deep breaths, it always helps me to stretch the muscles around my back by popping my back and stretching that area. ANYWAYS from what they told me its very very very uncommon to see a 15 year old healthy kid with a bad heart. So breathe big and thing happy thoughts!