Lidchick, the sewer gasses that you are smelling are being produced by the anaerobic bacteria, and other microbes, as a product of the natural digestion process of the organic matter in the sewage flowing into your septic tank. These gasses are vented into the plumbing/vent system of your house then flow to the roof vents, and then into the atmosphere above your roof.
Each plumbing drain in the house is protected with a trap so that the gasses cannot get into the house. If you smell sewer gasses in your home, then gasses are escaping through one of these traps, or there is a leak in the vent system. Proper sizing and placement of the vent in relation to the drain that it serves is very critical to the proper function of the trap on any water using appliance or fixture.
I recommend that you contact a Licensed Master Plumber to investigate, and repair the cause of this sewer gas leak. JWA