Getting a sulfer smell at start up.

I had to fill up in a shady side of town at a gas station charging 25 cents below average gallon. After about a quarter tank, started smelling a sulfer rotten egg smell. And of course the "Check Engine" light came on. Still runs like it did before. Could it be something in the gas? or is it just a coincident that the CAT. Convs are going out. 99 CVPI. I think I have 2 of them. Would they both need to be changed? How can I tell? Thanks all.

I think you'll find its the cats,,, you should pull the codes to find out why the check engine light is on,,, but I think it will have to do with the cats..