You are probably smelling hydrogen sulfide, aka rhino farts. This can be produced when cider is left on the lees for too long and when yeast are stressed. If you catch it early, you can splash rack a few times to clear it up. If you do this, make sure your sulfite levels are high enough to fight against spoilage. Unfortunately, if you don't get this until too late, you may not be able to get rid of it.
In your case, it sounds like the level is relatively low. Since it is highly volatile, you will catch a whiff of it when you first open the fermenter or a bottle of cider. As long as the concentration is low, it will probably dissipate quickly and you can enjoy your cider without out the rhino farts.