Peugeot 307 Anti Pollution Warning?

As I collected my car from passing its Mot the engine management
Light and the Anti-pollution warning flashed. It wasn't showing before and the car would've failed its MOT! The garage said to run it and it would go off and the only work I've had done recently is brake pads and discs! Am I that Unlucky that my CAT has gone hours after my MOT. The message comes on each time I start the car. It's an O4 Plate 1.6 Petrol. Please help.