Help, 550 lack of power?!

After re-fueling today, the car is running like it only has half power. It still revs out , is smooth and doesn't feel like it's missing or anything, but has the power of a Toyota Corrolla. Any suggestions?

 

Update.
I disconnected the battery, suspecting an electronic glitch. The power is now back, but the "Slow Down" light is flashing all the time.

 

The warning is flashing from a cold start, so I don't think it is the exhaust.

 

I did some research on the board. The car was not runing hot at the time, temperature was less than 80 degrees C (176 degrees F), but the power did come back after the car was left for an hour or so, even though the light was still flashing.
Further update, I just took the car for another run tonight. The light was flashing for quite sometime, even during and after warm up, but after a few "performance" runs, the problem now seems to have cleared completely.
Could this be due to bad fuel,? I refilled with 90 litres (115 litre capacity) and have now driven around 100kms since refuelling, so I am thinking that there was some bad fuel in the last tank.

 

Thanks Dan
Spoke to a tech this morning, he suggested the same. I am taking it in tomoorw morning to have it checked out.

 

The left hand bank O2 sensor was faulty, tried cleaning, but it was too far gone. The left exhaust was reading 200ppm hydrocarbons compared to 45 on the right bank. It's running like a charm now.