HI - so glad you got her back. I would take a poop sample in to the vet office to start, and a vet exam if you haven't done so. While she was gone it is possible she got into giardia, coccidia, and all sorts of worms, which can really do a number on them. Whips especially but they are all bad. They also eat up all the food on the dogs so they get no nutrition and lose weight!
What are you feeding her right now? It may not be helping as much as you think, because the ads we see on TV for dog food are as bad as the people food ones - maybe worse! I will tell you this, if there was no Internet, and just the Beneful commercials - I would think that was the best food.