Doing a little reading it looks like Notty has a tendency to throw sulfur at cooler temperatures. I just smelled my Irish red today and it smelled biscuity and great for a week and today, farts. Not a huge problem, they yeast still need to do their thing. I think maybe you cold-crashed it too soon after primary and stressed out the yeast. Maybe let it warm back up to the mid 60's? Mine it sitting at 60 degrees right now even and the smell is atrocious, even though the better part of primary is already done. Gonna move it up to a warmer area and see if it clears up, did the same thing with a perry I made and it worked out fine.