First, amaranth, I want to say I'm sorry for the predicament you're in. It sucks to be outted when you're not ready. I have some dear friends facing a similar sort of situation, and I know how anxious they all are about it. So my heart goes out to you, for sure.
I agree w/ Rarechild and YGirl. Your mum may already know.
Beside that point, though, you've missed your opp to dismiss the issue casually. When she brought up the message from the mysterious stranger, you could have replied along the lines of "Wow. Sorry that happened to you, Mom. John Smith, eh? How original. All kinds of kooks and wierdos on Facebook these days."
But now you've given credibility to the incident by saying you want to talk about it when xmakina can be present. (Why would you even need to discuss it if it's just a kook's prank?)
Having shown yourself this far, consider the possibility of coming out to her. She's your mom. She loves you, and she may be more understanding than you think. Of course, that depends on how open and tolerant your mom is.
Best of luck to you, whatever you decide to do.