The cause could be a parasite, such as Giardia, or Coccydia.(sp?)
It doesn't sound like Parvo or enteritis, as her appetite is good.
But yes, dehydration can set in when there is diarrhea, even if she's drinking. So make sure she gets plenty of water, and take her to a vet as soon as possible for an assessment.
I hope she's better soon.
First try to get the dog to drink water, dehydration is a first concern given what you describe.
A vet needs to examine the dog to get to the solution of the problem.