
Hi Emma,
There are many dogs that have a quiver or tremble in their legs for which a cause has never been identified. Is this what you mean by shake?
In some cases it can be a sign of pain, i.e. muscle spasms. In others it can be a sign of a nervous disorder. Or it can simply be incomplete mylenization of the nerve sheaths.
Sometimes it can be an affect of excitement, exercise or tenseness.
BTW, this is not a diagnosis, just a few ideas for you to ponder over while you take her to the vet. Good Luck. :-)
PS - In cases where these things are intermittent, it can be a good idea if you have a camcorder to take a video of her when it's happening to help the vet. ;-)