Sounds like a clogged vent pipe. Drainage systems need air to operate properly. This air is obtained from the vent pipe (same one that goes out the roof). If the fixture cannot get air from the vent, it takes it from the nearest fixture. When this happens, the water in the trap is sucked out, which allows sewer gas to enter the home. You need to find the blockage since sewer gas is neither good smelling or safe.
Sounds like you have a potential clog building help and before this happens and you have a major back up I would suggest running a closet auger to free up any build up. You will want to feed the tip of the auger gently into the drain opening and push down and crank clockwise until the cable resists turning once this occurs turn counter clockwise until the cable is as far down as it will go. Pull the handle up as you continue cranking. Now use a plunger and plunge and flush again to see if it runs freely.
If this does not work, you most likely have a problem deeper in your plumbing lines and at this point you should call a plumber or drain clearing service.