Toilet bubbling, foul smell, no known cause
It should be no surprise that the toilet dumps its contents into a sewer pipe inside your house, and the bottom of that sewer pipe is plumbed out into the city sewer system (or septic field if you have that instead).  

Less well known is that the vertical sewer pipe usually extends all the way to the roof, where there is a vent.   Gases created by food and effluent decay will vent there, instead of doing what yours is doing.  

It sounds like something has gone wrong with that system.