How did you save your yeast, did you wash them or did you just collect some of the slurry? If its just some of the slurry, its possible that the yeast had a little bit of extra time and were able to ferment a little bit of the beer that you brought with them, creating a little bit of CO2. That could also explain the sulfur smells, depending on the yeast. I'm picturing the whitelabs vials, I've had some of them foam up and hiss when I opened them and they weren't contaminated.
Of course, the other option is that they are contaminated, and its up to you to figure out if you want to give them a shot and risk the batch. Personally if it still looked like yeast and smelled like yeast (or at least smelled like yeast fermentation, which can be stinky), I'd go ahead and run with it.