Your vent pipe up through your roof might be clogged/blocked. Fill your sink up with water and remove the stopper - does the water drain slowly? If so, try filling up the sink with water again, remove/open the stopper, cover the sink vent (normally located closest to you when standing at the sink) and hit it hard with a plumbers helper. If that doesn't work, take a water hose up to the roof and blast the vents.
Check for odors around your toilet too. A bad wax seal will let in sewer gas.