I logged into my friends facebook account?

[.]

So my bestfriend gave me her facebook password a while back because she trusts me to not go through her messages or anything like that. Today I went on it, not sure why but anyway I just did. She's at school today and I'm at home, while I was on her facebook account someone sent a message saying "shouldn't you be at school?"
I don't know what to do now because she'll know I've been on her account and she'll see the message when she gets home, but if I delete that message the other girl who sent it might say something to her about it.
What should I do? I'm feeling very guilty at the moment.. I know I shouldn't have done it.