Sounds like you're getting a downdraft blowing through the fire pot. Do you have a power vent? Is the flame yellow or blue?
Is the damper working? Let me know, Tom

I am not sure what a power vent is, but the flame is blue. What is the power vent and the damper?

Thank you

Sheryl I know what Tom is looking for and I am afraid we would need to to tell us the color of the flame while you are getting the bad smell only. Flame should always be blue but will turn yellow/orange in a draft which will only happen when the wind kicks back up. Is it a natural gas smell, sewer smell or an unpleassant odor like from an oil lamp? A power vent would be a n electrically operated fan on top of the water heater that kicks in and pushes the burnt gases up your chimney or vent.