If the smell was only there for a short time, probably nothing to be alarmed about... all of my cars have given off a puff of pong at some point.
"Why do CAT emissions sometimes smell of rotten eggs?
This can sometimes happen when trace elements of sulphur in petrol form minute quantities of hydrogen sulphide on the catalyst. This generally only occurs when there is a rapid change of engine speed and load. It is a phenomena of new units and should reduce with age."
http://www.buypartsby.co.uk/catalytic_answers.php
"Q Why Does The Exhaust Have A "Rotten Egg" Smell?
A Many times on first start-up in the morning, the exhaust may smell like "rotten eggs". This is due to rich fuel condition with a cold engine. However, as the engine warms up, this smell should go away. If it does not, converter damage could possibly follow with extended driving."
http://www.catalyticconverter.org/faq/index.htm
If the smell is appears frequently & for prolonged periods, then you have problems.