Sulfur, 'rotten egg', smell from under car after driving. 2010 Aveo5 manual trans

I'm experiencing a sulfur smell from under my 2010 Aveo5 after driving. It is more noticeable if I rev the engine to higher RPMs. I don't smell it while driving, but only when I stop and get out of the car.

The car is a 2010 Aveo5, 1.6L, 5 speed, 13,500 miles
I use synthetic oil.
No check engine light.

I'm pretty sure I'm not burning up the clutch because I am experienced with a manual. Also the clutch feels fine.

I recently put on a K&N air filter, but I was smelling this smell even before I put it in. I've been smelling this for probably 6000 miles, it's just getting worse as of late.

Does anyone know what is going on/ have had this problem? Online says it could be the catalytic converter but this car is so 'new' that I wouldn't expect the cat to have problems yet.

Could be the cat.

well I am going to tell you something..., well two things it looks like now. That smell is produced by larger diesel engines when a burn off of the particulate filter is being done. But the Aveo doesn't have one, could be duplicated by a cat getting hot as Aveo suggested. I drove around in a truck for two years smelling sulphur, like burnt matches off and on...it turned out it was a sinus infection...so make sure someone else can smell it when you do LOL And good idea to make note of where your smell it, if your parking and smelling it in the same place everytime and nowhere else, could be a gas leak nearby.