Did your local area switch to ethanol, by chance? Ethanol blend fuels tend to get crappy gas mileage. My cars' mileage always takes at least a 3-4 MPG dip for 6 months outta the year when they use ethanol from October to May, then it comes back up when they go back to MTBE blend for the summer months.
Otherwise, I'd say something is amiss in your system - oxygen sensor, perhaps.