I would say that whatever's going on, she's either afraid she'll get into trouble, or afraid of disappointing both of you.
Regardless of whether they believe they can talk to you about anything and everything or not, there are some things teens may never come to us(parents) about.
She may be having some trouble at school that she's upset about, it may be embarassing, or she may be trying to deal with it on her own.
If there's a way you can find out about her grades without having to depend on her to show you, you should do it, and then maybe have a discussion with her that if you felt you could trust her to be honest about something so important, and what's caused them to become so bad so quickly, that you wouldn't have to resort to such a thing. Maybe talk with her counselor at school, and discuss ways that you can all help her.
And, if grades and keeping them up are important, it may be time for her to have some consequences to letting them slip until she feels like letting you in on what's going on so you can maybe help her.
Whatever happens, I hope she's okay and that things turn around for her soon. Kids go through so much. Just let her know she can depend on you both to be her shelter from the storm.
K.