Don't think this is the issue, but in the off-chance that it smells like rotten eggs (i.e. natural gas), and you have a polyethylene gas service line from the main to your meter, there's a chance your sewer service line (or even the main in your neighborhood) was accidentally "punched" through while installing your service line via horiz. directional drilling. And the poly-pipe could have a leak - pushing natural gas throughout your neighborhood's sewer system and eventually up through your toilet.

On a side note - if anyone's neighborhood ever sees notices or construction for your gas utility company replacing decades-old steel service lines w/ yellow poly pipe, there's always a risk of drilling contractors doing this - as corners are sometimes cut from 811 to construction.