Lately our dog has been urinating in her sleep. She is a very well trained 5 year old, and this seems to be a new problem. What could be causing her to do this?

It's possible there is an infection, bladder, kidney's... best for her to see a vet

I agree with melinda. Your dog needs a vet. Also I noticed you referred the dog to she. If the dog is spayed she may have a hormonal problem also.