Much of it can probably be attributed to the cold weather. (Actual air temp drop) I think that has a bigger impact then any change in the gas itself. Colder air is denser, meaning it has more oxygen content. That means you need a richer fuel mixture (more gas). While it can make more power as well, fuel economy suffers.
Also, people tend to let their vehicle idle more in the cold weather, also burning more gas.
Finally, in bad weather, even just rain, the vast majority of drivers get stupid, and drive needlessly slow, resulting in more stop-and-go traffic, and less fuel efficient driving styles. Personally, it really pisses me off that MY fuel economy can vary 2-3mpg sometimes, depending on how OTHER people force me to drive! Its only made worse in bad weather. If your little import crap-box can't go more then 5mph on a snowy road (or you just don't have the driving skill to do so), then stay the hell home!
So, as long as everything else seems normal (no change in drive-ability, no CEL,etc..), then I would blame it on the cold.
Oh, also, check your tire pressure. Low tires will also lower fuel economy. Tire pressure drops something like 1-2psi for every 10deg drop in temp. Don't hold me to those numbers exactly,but it is something you need to check.