Why do you suppose my 21 monthold is doing this?
She is on the last day of anti biotics, having had some horrible infection, probably due to something she ate.
But she seems to tremble often when sitting quietly alone or on my lap.

Karen

Sounds like she could still be having problems and being in quite a bit of pain, I'd be taking straight to the vet, hope all turns out OK.

Well she saw vet yesterday, and he was happy all was okay again, but I didn't mention it as it was so infrequent, and as  I said more at the quiet times. Will heed your advise and contact them again.
Karen

Could it be due to fireworks? Just a thought...

sounds like something still isn't right, so i'd be back to the vest.
trembling can also be because a dog is stressed, over excited or fearful of something, so fireworks or anything thats happening she isn't sure of could set this off.
my schnauzer does this when she knows we're going for a walk or she's about to be fed etc, she just gets wound up and excited.  

Not firworks as they are quite strict about the sale of them here.
Tonight and maybe tomorrow that will be it.
The trembling has lessened, just occasional.She is much much better anbd it is not obvious that its to do with anything, or that it worries her.
I am keeping a very close eye, again because of location, I can afford to watch as the vet is but minutes away.
Karen