Check the water lines leading to the washing machine. When they hooked
mine up they switched hot and cold. If they did that to you, and you
do a lot of laundry in cold water, you could be using a lot of hot
water and never know it.

And I'm sure someone else said check the temperature setting on the
water heater. If you're keeping a tank of water at 150 degrees that'd
use a bit more gas than you'd like to pay for.