My son experienced very similar pains around the same age. Because my son has a heart murmur, I was concerned enough to take him to our physician. He explained that, during that time of life, growth spurts will occur causing a stretching of the intercostal spaces (cartilage between the ribs). Stretching of these spaces can cause sharp, almost angina-like pain but it usually subsides after 10 days to 2 weeks. One way to check if this is the culprit, is to raise your arms above your head and stretch. If this exacerbates the pain or causes the same pains your experiencing, that will most likely be the cause. I am not an MD but this has been my experience. If it does not subside after 10 days to 2 weeks, seek the advice of a physician.
-Mary
Have you thought about anxiety as being the cause? Often times anxiety attacks can feel like something is happening with your heart. What gives me a clue into thinking it may be anxiety is that it happens in the middle of the night, and only happens every so often. Also, anxiety is definitiely common at your age. Generally if it was an issue with your heart you wouldn't experience left sided chest pain, but generalized chest pain more centrally located. It's often a misconception that the heart is on the left side of the chest and therefore when you have pain there it must be the heart. A good portion of the heart is actually towards the middle of the chest. Often, pain on the left side of the chest actually turns out to be related to the stomache or esophagus, as that's where they connect. What I suggest is you go to see your doctor, that way he/she can give you a physcial exam to rule out any medical issues. If there is nothing medically wrong, the culprit may be anxiety. Your doctor can then talk to you about options on how to treat this. I wish you the best of luck and send good wishes your way!
Sara RN