Dogs may shake due to fear, pain, fever, spinal issues, liver or kidney problems etc.

In this breed ear infections are pretty common and pain from that could cause shaking.

If this is very sudden sometimes a seizure can cause this too.

If your dog might have gotten into anything toxic, appears dehydrated (see if a pinch of skin over the shoulder blade stays stuck in a tent form or flows back down normally when released), has gums or tongue that look white, blue, gray or yellow tinted where they are normally pink, has a slow color refill if you press on a pink area of gum or tongue, or is becoming lethargic and non responsive to you then you may want to get the dog in to see your vet or an emergency vet if your vet is not available.

This site may help you locate an emergency vet if you need one

http://www.vetlocator.com/hotline.php

Hope this helps you!