First off congrats on your 30 days clean!
To me it is a good idea to tell her about it. Sometimes good to let all out. Even when it's hard times you are going through its good o let it all out.
I would say (I never done this before so idk what's the best) just say can I speak to for a minute or Can I tell you something important whenever you have the time. The chances are she will say yes or something like that. Or is she asks you if you are ok then be honest and say no, can I tell you something alone? Then when she asks what's up just say it directly it's a lot easier to just say it and not to make it complicated!
(speaking from experience)
Other than that it is a really good and brave thing of you to do!
I never had the courage to say something about
SH to an audult. Someone was suspicious of me
SH and told the school councelor and then they found out from there!
but still I'm proud of you for even considering it!
Good luck for whatever choice you do!