People in here are all telling you the wrong thing. Your problem is your battery that is emitting sulferic acid from its vents that is why you smell the rotten egg smell that is what sulferic acid smells like, the reason your car is harder to start is because its not fully charged and you probably need a new battery and the fact that you did a tune up on your car has nothing to do with it most likely all it is coincidence. so test your battery and you will see it is probably not up to specs but also make sure that you have not crossed any wiring like from the alternator or anything else that could be causing the battery to heat up.
Thanks!