This almost always seems to happen when coming down from vaped spice. Like it's been said, though, it's not uncomfortable. It almost does feel more like vibrating than shaking.
With aya, the perceived body temperature seems to fluctuate wildly, seeming to be freezing one minute, and boiling over hot the next minute. When cold, massive shivers happen, different than coming down from vaped spice. Not sure if that could be chalked up to vasoconstriction or not.