The smell of rotten eggs is most likely coming from the catalytic converter. It may be telling you that the fuel management is running rich, possibly a problem with the oxygen sensor. Repeated short trips or running cheap fuel with a high sulfur content can also cause this. As can a faulty catalytic converter.
In olden days before O2 sensors and closed loop fuel management this was common. If it is only occuring occasionally and you run a lot of short trips I wouldn't be concerned. Otherwise you should have it checked out. Running a rich fuel mixture over extended periods will shorten the catalytic converter's life and they are not cheap.