Or it could be pain.
My old dear girl would shake when she hurt.
Has the dog been vetted lately?
Ruby will shake when she is excited (usually if she sees prey) or cold…hopefully that is all it is.
When I was taking care of a sick Jack Russell (congestive heart failure, back issues, and arthritis in one rear leg), he would shake and the vet said the shaking was due to pain. He ended up having acupuncture and it helped the shaking considerably.
This is the 1st time someone brought this up. Duke's front leg will shake sometimes. He's young, nearly 2 yrs old now. But it's been occasionally happening since he was a pup. He's very muscular and I think it happens like a muscle spasm. He is usually sitting at full attention. He could be anticipating something good when it happens - nerves. I never thought he needed to see a vet about it. Though I have held his leg until the shaking stopped, to see if it would stop - and it does.
thanks for all the responses. it is really chilly here in ohio, so that might be it. he shows no other signs of pain, but i'll watch it more closely and see when and how often it happens. when i put my hand on his chest and pet him it stops. Sharron- the last time he was at the vet was a couple months ago.. and he's in good health, no problems that we know of..
My b's wear sweaters from Sally Wallis.
They are great and keep the dogs warm.
Let me know if you want the site, I will send it to you.
I love the fact that they make my dogs happy.
Tayda does this… Lenny doesn't. I think Tayda does it when she is cold or nervous...
sharron- i would love to check out those sweaters! thanks!!
You can find the on E-bay…. from Sally
Check out this site…My b 's and l love these sweaters.
http://www.zandebasenjis.com/warmth.htm
thanks sharron! they look nice and comfy. i like how you dont have to put the front legs through holes or sleeves. I'm def. thinking about ordering one of these!
Mine does this but clearly when he's cold (like sitting out on the deck after the sun goes down). It sounds like your little beastie is just chilled - but if he does it even with covers/sweaters I would have it checked by the vet.
We put a sweater on Dash in the winter and a coat for him to go outside. Even with the sweater he crawls under the covers or lays by the heater for warmth.
Bought She-Ra a sweater last trip to obedience training. She was shaking and obviously cold…the rest of my group helped find one that worked for her and now she's all dolled up. The wife thinks I should have picked up something more feminine, but I'm more about function...it's a good warm sweater, it was on sale, and it's her own fault for sending out a guy to buy clothes! 
Bought She-Ra a sweater last trip to obedience training. She was shaking and obviously cold…the rest of my group helped find one that worked for her and now she's all dolled up. The wife thinks I should have picked up something more feminine, but I'm more about function...it's a good warm sweater, it was on sale, and it's her own fault for sending out a guy to buy clothes!
Good for you! Would love to see She-Ra in her sweater. I'll bet your DW will put a new sweater on her next shopping list. Soon She-Ra will need her own closet - hee hee 
My b's love the Sally Wallis sweaters.
They come over "ask" to have them put on in the cold am.
