:(
I'm really sorry to hear this.
She sounds like she might be having an identity crisis.
Being a teenager you feel like you have to prove yourself to other people, and whether that means bragging about having sex or smoking weed, kids will do anything to fit in.
Trust me, I am one.
Maybe strike up a conversation with her,
Don't make it obvious you've "snooped" around,
But just ask her if everything is ok.
Try and have a deep conversation about your feelings in general and she might come around.
But for now, she might just have to make her own mistakes.
She'll realise it's stupid to lie in order to fit in, one way or another.
Just try and approach her as a friend, rather then an authority figure.
She'll be most likely to realise that way.
Hope this helps.