Let me start by saying that DC is still tiny, so I'm not sure what I'd do with a teenager, but...
My mum found notes when I was 14 between me and a friend talking about a girl I fancied and how me and a friend were going to go to a gay bar to see what it was all about.
Mum made me sit down and "explain" it, mainly with her having a go at me for a. writing notes in class, b. putting myself at risk by venturing "into a community I knew nothing about" (rubbish on so many levels) c. keeping secrets from them etc. She also told me the (older) girl I fancied would think I was a "silly little girl".
The overwhelming feeling I had were crushing embarrassment, which meant I would never talk to my parents ever again about personal matter; complete betrayal that she'd read personal notes, she also "found them cleaning" (she probably did, but I didn't believe it at that age) I thought that even if she had found them by accident she shouldn't have read what were obviously private notes; and her disapproval of my feelings, which led me to conclude that my mother was a hypocrite and anti-lesbian, again I have never spoken to my parents about my sexuality since, even though I am now married.
That was 11 years ago and it completely destroyed any trust that may have existed between us.
I firmly believe that if she had taken that knowledge on board and then put her energy into building a proper relationship with me, one based on trust, then I would have spoken to my parents about my issues as a teenager and she would then have known what was going on (or at least more than she did) and I wouldn't have worked so hard to fence them out of my life.
It must be awful to have read that, and know it, but for know try to reassure yourself that they mentioned condoms. It might be made up, the things I'd learnt pretty much all the terminology to go with sex by that age, even though I hadn't done any of it.
I would forget all about the notes. They don't exist. But maybe try to talk with your DD about these things, and as cyrilsneer says if you show her that you can talk without going off the deep end she'll hopefully trust you and open up even more.
I would have loved a mum I could have confided in as a teenager. And one I could have confessed everything to and cried on knowing that I would just be loved and not condemned.
Sorry if that's no help though.