Yes, certainly is possible that she could have had a miscarriage and no one knew -- or it could be that her period was just really late, she thought she was pregnant, told her boyfriend she was, and then her period came and she thought she miscarried.
I am glad she is now on birth control. I am surprised they gave it to her without a pelvic exam?
As for the diary...yes I think she wants to talk to you about and doesn't know how. At that age I would have NEVER left my diary anywhere, in fact I stopped keeping one because I was too worried my Mom would find it. OTH you can't just say, of honey, I was reading your diary - AND.... I think you will have to casually drop things to her as informational. Like if you see an ad on tv about teen age pregnancy, maybe voice your thoughts on it and how if it ever happened to her you hope she would feel comfortable enough to come talk to you about it. Or watch the news and bring up the controversy that is going on with the morning after pill, and see where the conversation leads. I think just being available, and letting her know you are open to talk is the most you can do.
The other day, taking Matt home, and somehow we got into this really long conversation about sex, birth control, and responsibilities. It started because I was talking about a guy friend I have, and what "trouble" he had gotten himself in. After an hour, Matt and I were still discussing this topic. His views were SO appalling I almost drove off the road (If I got a girl pregnant, I would just ditch 'em). I tried not to be judgmental (it was hard) - but I looked at him and said - "why so a kid can grow up like you without a dad? You KNOW how much that hurt you and affected your life." His eyes got as big as saucers. I swear, he had never thought beyond himself on this topic - and I was SO glad that somehow this forum had opened up for us to be real. And I can only hope I gave him some serious food for thought
Anyway, I will stop rambling - hugs. And I hope she feels better from her other stuff soon.