Honestly, you're the only one that knows your mum. Does she have bigotry issues? Has she ever said anything negative about LGBT community or persons? Has she ever told you she loves you no matter what? Has she ever come to your defence?
It's up to you to decide when and what to tell her. Complete honesty is ALWAYS the best way to go. As Gennee said, give her time afterward to digest.
I am not trying to push you either way, please don't think that. This is the major issue that will effect your lifelong happiness. You know who you are, is it time others in your life knew? You're still young.