Why Do I Sometimes Start To Suddenly Shake?

Sometimes, this feeling comes over me and I start shaking for only a couple of seconds. It doesn't happen too much, maybe once or twice a day, and never around the same time. It's very random. It's not like only my hands shake. My whole body shakes. I've been this way ever since I could remember, but it used to be very very seldom, like once a month. I don't know why it's more frequent now. When I get the shakes, I don't feel cold or feel sick or anything, I just feel the need to shake my body, almost like a dog when they are wet, or like when you're really cold and you shiver. What could this be?