I start getting random pains behind my left shoulder blade,and then my entire left arm gets numb.heart or maybe muscle related?

Most likely..: In your age group there is a very low chance of having heart disease unless you suffer of illnesses that pose a risk for this problem. If you are otherwise healthy then this is most likely muscle related and the numbness in your arm could be related to a nerve irritation at the level of the neck or the arm/hand itself. You need evaluation for diagnosis. Follow up with your doctor.