What Causes Ache Below The Shoulder In Pectoral?

I have an ache just below the shoulder in my left pectoral. When I make any movement that causes the muscle to contract, such as sitting up from a reclining position, it becomes a sharp pain. I am guessing that it is caused by a pinched nerve in my neck, but this is only a guess. I am 73 and do not recall having done anything to cause this.