What are you feeding them, feed and treats? Molting is hard on them and can really take a lot out of them. Being "fat" isn't good for a laying hen, but I know that doesn't necessarily mean that she's actually overweight. I have a hen that lost a lot of weight before I realized it and she's still...
I'd keep a close eye on her weight because diarrhea can cause them to lose a lot of weight quickly. You might want to start her on some probiotics, along with what Mare mentioned.