Decline Of Gas Milage Then Back To Normal

On the Deville the SES light started coming on and off code E44 and then the gas mileage done a nose dive, after a week now the mileage has returned from as far down as 14.8 back to 21.8 and the light has not come back on for 2 days now thank goodness!! what in the world would of caused this? Kraig