This happened to my mom and stepdad. It was fine for the first couple months they lived in the house, then WHAM, bad smell time. The previous owners had not kept up with the proper maintenance schedule on the septic tank. This is really important, as I understand it, since if it overflows into your leeching field (spelling??), you'll be in some expensive trouble. I'd get the septic folks out there real fast, just in case it's NOT the water softener.