It is not uncommon for the cat to stink a bit when you put the engine under high load (going uphill, overtaking, etc) its caused by the honeycomb inside releasing sulphur, hence the rotten egg smell, but depending on your driving style, it shouldnt last long.
If the fault light is on though, it is likely that the ECU has stopped listening to a sensor that it believes is faulty/incorrect and will substitute that information with a default figure, and as it is always safer for the engine to be slightly rich (too much fuel) than too weak (too little fuel) it will run a little more fuel than is required and this can cause a smell on high load.