D.T.
My little one does this if I wake her. I think it might be the transition between sleep and awake for someone so young. Little ones usually sleep very soundly and waking from a dead sleep can be rough. I think it could be caused by the nerves suddenly going off to wake and move the body. I'm no expert but I would think the shaking would be the same as the shakes I get when I'm over tired. Nerves all misfiring trying to keep me awake longer then I should be sort of thing. I don't think it's damaging her in anyway, but I can't imagine that it's very pleasant.Is there anyway you can keep her from falling asleep on the floor? That way you don't have to move her and wake her. If you have to move her try moving as softly and slowly as possible if she begins to wake/shake talk to her soothingly, hold her close and try to get her to relax a little.
I hope this helps.