That's good, I don't think stress either if she is laying good and they will have times when they don't lay to rest their bodies. So try the Blue Kote and some Vitamins what is their day like do they get out and forage around or are they confined to a run? It could just be she has overly preened herself and the cycle needs to be broken. She'll be a little purple for a while but if it breaks the cycle then it's worth it. I have had many a chicken with purple on them where someone got carried away with the preening. You checked her over for mites?She has laid her eggs like normal with the exception of a few colder days, they're large over 3.0oz eggs, very strong, and hearty feeling. Knowing my ducks and knowing their personalities she hasn't been acting abnormal at all. Neither one of them. I don't think it is stress but of course I could be wrong.