My check engine light has been on. I had it checked by my mechanic and was told no codes showed up. This morning the check engine light started blinking.
If the car is working perfectly fine. Could it be that you did not tighten the gas cap when you refilled? It looks like this is a very common problem. You may even need a new gas cap.
See this for more info: Check engine light is on, but no codes?
I would go to a different mechanic next time. :)