You only feel your liver or kidneys when they are not functioning correctly. You feel your heart, i assume, because some sort of feedback mechanism is in place in order for the organism to know how hard he/she is working, the response to stress, and also because it is a muscle that impacts vessel pressure all over your body.
I'd suggest getting a stress test, just to be on the safe side. A lot of mineral imbalances can effect the heart, but at least in this case they can rule out any ischemia (areas in the heart that are not recieving adequate oxygen during rest or exercise) or arrhythmias.
I'd also try to avoid most stims as well...stick to moderate coffee if you need a booste.
Br