Cold weather often brings skin problems such as dry skin, so you might be right on the mark about her lying near the fireplace. The white flakes are most likely normal cat dander (which is the stuff that causes allergies in humans.) There are a number of commercial products available for both topical and oral treatment of dry skin.
You could also try adding a bit of oil to kitty's diet, particularly if she is eating a straight dry food diet.
Frequent brushing or combing: two or three times a week for shorthairs and daily for longhairs, also helps in keeping dander and shedding at a minimum.