My brother recently unclogged my bathroom sink and pulled out all kinds of hair. Ever since then, every time I turn the water on, I now have a horrible "rotten egg" smell. I have no idea what is causing this because it never happened before. Help !

did your brother pour liquid drain opener down the drain?
some of the stronger cleaners contain sulfuric acid which smells like rotten eggs...

No nothing was poured down the drain at all.

and the smell is not coming from any other fixture/s?

No, it is definitely coming from only the one sink - no other sinks.

Does the sink have an overflow hole? Maybe the hair was plugging the overflow drain in the tailpiece and now with that open, air freely flows through the overflow passage, carrying the funk smell with it. If you have an overflow, try spraying foaming bathroom cleaner down the hole.

Approximately once a month I put a stopper in my lavatory drains and fill them up to over the overflow. I put my thumb over the overflow and add 1 cup of chlorine bleach into the sink then remove my thumb while letting the water continue to run. I have to adjust the water so it does not overflow the rim of my sinks. Leave the stopper in and every 10 minutes or so I come in and take my hand and pull water to the overflow. This bleach cures the smell from the bacteria inside the overflow. I also have a ball tube that they fill batteries with at the old gas stations and I use it with the bleach. I put a beveled washer that we used to use for 3/8" tubing to connect the old angle stops to the lavatories. I hold the washer on and it covers the hole enough to enable me to put pressure on the blow out of the bacteria. It works pretty well. Just be careful with the bleach or you mess up your good clothes. Try it...it works.