First, you need to be checking your mileage based on how much fuel you are using, not when the idiot light is coming on. The procedure is pretty simple.

Fill the tank.

Drive until you need to refill.

Fill the tank again, taking care to try to fill the tank to the same level.

Divide the miles driven by the gallons of gas.

Right now you have no reliable metric. You may or may not be using more fuel. The light is not an accurate measure.

Second, nearly everyone will get worse mileage in the winter. Not only does it take a bit longer to warm up, but the ethanol content is higher in the winter.

Also, driving conditions may be less condusive to good mileage in the winter, even a California winter.