It's bacteria in the water heater. Bleach it and it will go away for awhile. You might also increase your water heater temperature. I have 3 water heaters running from the same well. The main house has no issues. My in-law apartment (less water use) has a sulfur smell that I can fix with occasional bleaching. My new outdoor patio sink (2.5 gallon with minimal use) will bowl you over with sulfur smell. I'm experimenting with the bleaching schedule and raising temperature to cure that.
