I shake internally almost all of the time, sometimes it is better, sometimes worse but it is always there. If I am shaking externally it is usually going to be when I am in or close to being in panic mode.

Growing up showing any kind of emotion was a weakness, it made you a target. I have always figured that the internal shaking had to do with that. A "physical" reaction that was not visable. A way for the body to react and work off excess adrenalin or something.