Could be an exhaust leak, faulty o2 sensor or lambda sensor on the downpipe.Are you running 99 octane fuel?
Has the light remained on??
Apparenty if you do lots of short journeys , this car occasionally happen as mine has recently done this, but only did it once, and after shutting down, and restarting ,hasnt come back on in the last fortnight.
Either take the car back where you had the exhaust fitted to get it checked, or get some data logs done from someone with VCDS to assist further.