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If nitroglycerine is helping relieve the pain, then in all probability, it is angina. Even if you are 17 years old, you need to get this investigated by proper TMT, ECHO, and maybe even angiography, and other tests like kidney and liver function, heart enzymes, etc.
This is something you should not ignore. It can also be a hormonal thing causing poor exercise tolerance. Until this is sorted out, please avoid exercising.
Do discuss this with your cardiologist and get yourself examined. Take care!
Your age, the relationship to physical exercise, etc. suggest a thoracic
outlet or scalene syndrome. Have MD examine neck muscles and ulnar nerve
function in hands. Reduce, if possible, exercises and heat to front of neck over collar bone,, Appropriate pain pillls and muscle relaxers will help also. You are too young to be in USMC- What about school? Coop254