I would call and talk to the vet. The vet may say to give the medicine another day or 2 to work, or to bring her back in. Do you know what kind of medicine the vet gave her? Was it an antibiotic or something such as Imodium for controlling diarrhea?
Although there is a bland, recovery diet in the ''My dog won't eat.'' sticky,
http://www.askmehelpdesk.com/dogs/in...tml#post254171 she is better off if you follow your vet's instructions. Worry more about controlling the diarrhea than her eating. She should do OK not eating a day or 2, and as long as she has the diarrhea, eating won't do that much good.
A sudden change of food can cause digestive upsets, but I don't think as bad as you describe.
As she recovers, you should find the ''What does a puppy need?'' and Housebreaking sections of the sticky useful too.