Why is water heater making the water smell like sulfur?
A sulfur smell is usually a sign of sediment in the tank. This is common if the tank is left on during long absences or low volume use. A spent anode can contribute to this smell, since other parts will start to corrode once the sacrificial anode is spent.

You should purge the tank by draining it from the bottom valve with the water supply and power turned off. You may be shocked by the gunk that may come out. Once the tank is empty, turn on the water (no power) and partially fill the tank allowing it to continuously drain. Repeat this flushing until the waste water is clean and fresh smelling again. the anode is often inserted in the top of the tank and can be removed with a wrench. A new one can be inserted, seal with some plumbers putty or teflon tape.  Once you are satisfied you have purged and flushed the tank, turn on the water, fill the tank and run the hot water faucets to clear any remaining bad water out of the feed pipes.  After all this is said and done, turn the power back on and heat the water.