one of your injectors is likely mal-functioning.
It may be that it's remaining open or semi open at all times. This will cause the engine to pump exhaust gases and fuel. This will mean that fuel is pooling in front of the cat. This would make your car a bomb, literally, waiting to go off. Which seems unlikely since you've been driving it a while now.
The engine should be running a bit poorly for this hypothesis to be correct.
The only other options I can think of are a fuel leak in the air assist part of the lower manifold, which has caused it to enter the intake.
The engine should be making less power, and the line air assist lines (either side of manifold, meeting in a plastic box above the thermostat) should be full of pressurised fuel when the engine is running.