Need help!!! Check engine light after coolant flush!

The check engine light just came on as I was doing my coolant flush. Here is what I did:

1. Drained the coolant at the radiator plug on the lower side of engine.
2. Ran water through system with car off and the plug off.
3. put the plug back in, ran water into the system and topped it off, started engine til the thermostat opened.
4. Drained the water out.
5. Put the plug back in and added coolant, started car, and as the car was warming up the check engine light came on

Any ideas? did I do something wrong? I have to drive home to go to work (i'm out of town) and i really need to figure out what happened, please help!!!

I went into autozone and had them pull the code, it's a P1131: Lack of bank 1 02 transitions - Lean


Any suggestions?

Yeah man, I don't know why that code would get pulled after a coolant flush...I just drove it for an hour to get home and the car ran just fine...I'm hoping it's just a fluke - if it doesn't go off on its own I'll try to reset computer.

It's possible it could be the o2 sensor acting up...anyone have any experience with that code?