If actually on your left side, it may just be a strained muscle.
Your heart is almost in the center of your chest behind the sternum
Depends when the pain comes. It should be accompanied by some difficulty breathing
to be the heart.
Get a heart rate monitor
, and see if your heart stays at consistent rates when exercising, or jumps around for no reason. That would be arrhythmia
, for which you should be seen by a doctor.