What causes a buildup of energy in a person?

Sometimes at what seems like random I have the feeling that something is building up inside me. It's weird to explain, there is no pain and no shortness of breath afterwards. It could be described as the physical manifestation of something like excitement or a build up of adrenaline, except it happens randomly without any cause that I am aware of. Often times when it occurs it feels like some sort of spasm when a muscle or multiple muscles tighten or straigten to release the "energy". Anyone know what this could be?