Funky smell?

[ol'sport]

I'm attempting to make cider using Louisburg brand apple cider, which claims to have no preservatives or additives. (Still pasteurized, though) I added about a cup of brown sugar to aid in the fermentation. It's been brewing for about 5 days and when I uncapped it to sample it yesterday, I noticed a distinct sulfuric smell. Very unappetizing. None of the other juices I've EZ capped have had this problem. Any idea what I did wrong? Or is this nothing to worry about?
Thanks!