Most ikely you are suffering from Soldiers heart, which is otherwise referred to as Da Costa's syndrome, which was colloquially known as soldier's heart, is a syndrome with a set of symptoms that are similar to those of heart disease, though a physical examination does not reveal any physiological abnormalities. In modern times, Da Costa's syndrome is considered the manifestation of an anxiety disorder and treatment is primarily behavioral, involving modifications to lifestyle and daily exertion. It is also variously known as cardiac neurosis, chronic asthenia, effort syndrome, functional cardiovascular disease, neurocirculatory asthenia, primary neurasthenia, subacute asthenia and irritable heart. Symptoms of Da Costa's syndrome include fatigue upon exertion, shortness of breath, palpitations, sweating, and chest pain. Physical examination reveals no physical abnormalities causing the symptoms.
What comes to mind is could an artery be spasming. Sometimes arteries are only 30 percent blocked and the artery will start spasming. This feels like a heart attack. Pressure, pain and sometimes the back can actually shake. Doctors have a hard time diagnosing this. An angiogram will have to be done to diagnosis this and sometimes that does not even catch it. Be aware of your symptoms and if it continues or gets worse, request an angiogram.
On the other hand, it also could mean you have an overactive adrenal. This is not the kind of adrenal you get from being scared, but the kind of adrenal from exercise, chocolate, caffeine and licorice. This is not harmful but produces symptoms of heart racing at times, breathing problems from heart racing, inability to climb stairs without heart racing and feeling out of breath and faint, aware of heart beating and pulses.
If you can, increase your calcium intake to see if this helps. Make sure to get plenty of potassium from organic bananas and potatoes. Increase your Omega 3 if you haven't already done so.
Hope you feel better.