It may be allergies. I have a dog that is allergic to dust mites, so the winter is worse for her (when the house is all closed up).
OMEGA FATTY ACIDS! Supplementing with Omega Fatty Acids is a great way to battle skin issues. In addition it is good for the heart and joints. There are veterinary omega chews called "Catalyst" or you can get fish oil capsules from the store and put it on their food. You can switch to a diet with a high omega content. I use Royal Canine Skin Support, but any fish based diet SHOULD be high in Omegas. The omegas will take about a month before the "kick in."
You can also bath her more often using special shampoos. I really like Epi-soothe (veterinary product). I works extremely well. When it is really bad, I bath them once a week, but can usually go every other week or once a month.