Simple…the chi is starting to go through your chi channels, and there's a bit of friction going on. Nothing special, unusual, or serious. Nothing to be concerned about. Nothing special to you in the whole world. Nothing that hasn't happened to thousands of other people.

No, it's not the devil shaking you. No, it's not a demon. No, it's nothing supernatural.

Boy have I heard all sorts of crazy explanations like that — even coming from monks!

Sometimes, of course, you could just be releasing pent-up nervous energy. In that case, let go, let it come out and relax. Don't encourage or discourage it but let it express itself and then die down. Maybe you just needed a break for it to come out (in repressive societies/regimes/situations people sometimes need a release valve for their stress, and exert themselves in extra movements to effect that release).

People have a tendency to turn everything from cultivation into something mystical, mysterious or supernatural. Don't — that could not be further fromthe truth. The reactions you experience from meditation - heat, cold, shaking, vibrations, etc. - all have scientific explanations. Don't reach  for the abnormal or superstitious. In spiritual cultivation, you cultivate wisdom and levelheadnedness.

To learn about all  the reactions people can experience from advanced meditation efforts and their explanations, I've never found a better book than Tao and Longevity, by Wen Kuan  Chu and Nan Huai Chin.