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.