Why does my water smell bad, but only in the bathroom sink?
If it’s only in the bathroom sink, then it must be an odor coming from the drain. Run you some Liquid Plumber through it, followed by a lot of hot water. Should get rid of the odor. If not, get you some vinegar and pour about a quart in it. Let it sit overnight. Then rinse with a lot of hot water. If this don’t work, pull the P-trap and clean out everything you can get to.