I take it you're new at the sex game. I also replied to your other post, and I experience this aswell as do many others I am sure. Just the whole ordeal of having sex has an affect on your body, causing you to shake. It is natural, it's mainly just the pumping heart and emotion that sends the body into a frenzy. Shaking is normal...For about 10 minutes or so...If it goes on for hours i'm not sure.