1995 Dodge Neon smells and power

Smells problem
1995 Dodge Neon Automatic 178000 miles

Just recently had an oil change and fluids checked as well as the o2 censor replaced with used one. Just yesterday my check engine light came on and every once in awhile when you are driving a weird smell like rotten eggs will come out of the vents. It also feels like it looses a little power or runs harder when you are driving interstate or higher speeds but doesnt do it all the time. What could this mean?