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.
Shut the car off! A blinking check engine light can quickly destroy an engine. Possible causes include leaking fuel injector, broken ignition coil/wire/spark plug.
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. :)