It might also be a problem with your plumbing vents.
Once in a blue moon, the vent on my roof gets buried in snow.
It usually thaws quick enough because there’s a bit of warm inside air leaking up, but if we get lots of snow at once, sometimes it happens.
If the vent is plugged and air can’t get in from the vent to allow water out l, it will either start to go uhhh… glug-glug, or sometimes that just siphons one of the other traps to allow air in that way, which then also allows smelly air through there.
You’re likely to not notice the gluglug or the siphoning of another trap happening elsewhere when you flush the toilet.
I’m not a plumber, but that’d be my guess.