In a human, what you are describing would be called SAD: Seasonal Affective Disorder. Maybe she'd like a sunlamp to give her a warm spot to laze around in? Here's a link for humans; some of them may work for her, too:
How to Treat Seasonal Affective Disorder Without a Sun Lamp - Associated Content - associatedcontent.com
I live in Florida ... it's not something we have to deal with (many OTHER things, but not SAD!). But when it's rainy and the dogs don't want to go out, a nice 10 minutes or so of "fetch" always seems to fit the bill ...