Why does my cold water smell like sulphur suddenly?
I have to somewhat disagree with Ed, which is rare. When a problem like the OP described that springs up suddenly, it's probably not a water quality issue.  From wells, the water quality (or lack there of) is pretty consistent.    What the OP is most likely to have is an anaerobic bacteria colony somewhere in the cold water plumbing.  Sounds really nasty, but there is a fix. It's not uncommon, esp. when you have well water. I have had a similar experience at my home.  You have to bleach the system. There are various methods to do this. Ed Beal described one. Others involve pumping / circulating a diluted bleach solution thru the cold water piping. The most important part is letting it sit for a few hours for treatment.
This is sort of gross, but just want to warn you that after treatment, and you turn the water back on to flush out the bleach, the cold could be VERY discolored. In my case I had a couple of faucets where the water came out BLACK! Totally black. Even after flushing for a long time, you may notice a slight chlorine taste for a few days. ....nothing to worry about, it will go away.