A sulfurous odor or "rotten-egg" smell is commonly caused by the breakdown of sulfate in your hot water heater. Regular flushing of your hot water heater will eliminate this problem. Clogged/dirty drains are another cause of odor problems.
To determine if this is the problem, fill a clean glass with the water, walk away from the sink and smell the water in the glass. If you do not notice an odor in the water in the glass, the drain may be the problem. To correct this problem, clear the drain of any debris or other substances that may cause an odor.