The green OD light on the dash is (most likely) on because you pressed the over drive disable button on the side of your shifter. (That's where this button is on my Contour. My green light is on the right though.) When that is on, the trans will drop out of over drive. Sounds like it down shifts one gear. That may be desirable on a long upgrade (hill) or pulling a load. That takes some of the load off your transmission. Makes it's easier on the engine too.
If you were smelling rotten eggs, that's often the smell out of the catalytic converter when it's over loaded or the engine is being worked exceptionally hard and the cat converter has a huge job to do. (Cleaning up the fuel the engine did not need in the first place.) Not that uncommon. I often smell it when someone is pushing their car hard and passing me.
Might not be a bad idea to warm the car up and check the transmission fluid level. Don't run it low. Not a bad idea to have the trans fluid changed. Don't think I ever heard of a fluid change hurting anything.
