pain in heart, why?

I never had a robust heart. I remember even in childhood when I was playing or running my heart was going into fast beat (tachycardio) and then when in my early 20's was doing martial arts and had to stop as felt sever pain in my heart (not chest pain) and now in my 50's I think that it is weaker now since even a fast walk home from work (especially when I haven't slept well the night before or haven't eaten well with high protein food) then I feel some pain in my heart (again not chest pain). Anyone has experienced similar symptoms or anyone has any idea what it is and could it be life threatening? My reading are average blood fat, normal blood pressure (with medication) and borderline blood suger (pre-diabetic). never smoked or use drug or alcohol. Everything else normal. Been to doctors. Nothing came out of it except the tachycardio of not dangerous type. Thanks in advance.

They tell me that the pain I feel is not from my heart but I and only I know where the pain is from and it is from my heart. In fact whenever there is a sharp pain, the heart suddenly beats like a hammer that instant. My pain related to lack of sleep, stress or lack of food (pretein rich food help my heat significantly), rather than blocked arteries, I think since when I sleep well or eat well then the pain goes away for weeks and whenever one or especially several nights unable to sleep or having stress or go on severe diet it comes back. If it was blocked artery, it wouldn't constantly unblock and block with sleep or lack of it.

Anyone has a similar experience that when not sleeping well several night or not eating enough meat or stress feeling pain at heart??