You probably checked this out already, but let me mention it anyway. I was at a service station getting gas with my son. It looked like rain, so while I was putting Rain-X on the windshield he finished fueling the car. A ways down the road the fuel guage started flashing and the CEL came on. The code was something like "evaporative system leak-gross". It turned out that my son forgot to put the fuel cap back on! I bought a new cap, cleared the fault, and it's been fine since. I doubt you forgot to put the cap back on, but maybe it's loose or has a leak.