Pain hovering over my heart...

The pain started today. I thought it could have been from my hike yesterday but I've hiked mount Pisgah so many times in the past and never before have I felt this....it really hurts...not enough to push me to tears, which you all know takes something severe to do that, but this morning it was as its worst and it was severe enough I did nearly drop my head holding back tears of pain...even now it still really hurts, I can feel it beating and it hurts. I went to the school nurse and asked her if she knew anything about what it could be and her first thought was "...could it be stress?" Stress? No...you guys know that no matter how much I do that would drown others in stress I'm a very relaxed person and avoid stress wth ease even in the stressiest situations. So not that. She brought up muscle pains, I said it could be a possibility of my hiking, and she said only if I had to work my arms to scale something like rock climbing. I didn't do that. She suggested maybe the strain of my side bag I carry around---wrong side of the body. I carry my side bag on my right shoulder, not my left...she was out of ideas and so am I. I'll be alerting my parents about this when they arrive to grab me to get my license, and we'll keep a close feel on it to see if it's only today or if it prolongs. It's definitely not a heart attack because I'm far too young for that so she threw that out the window. Any other logical possibilities to my pains?....and please keep in mind my age I'm not prone to anything anyone 30 and over would be prone to...