15 yr old deletes her texts msgs and im history every night before she goes to bed.?

I finally asked her why she was deleting, her answer was i don't know. I ended up grounding her until I can figure out what to do next. I don't know why she feels the need to hide everything. When I go to check her ims or text, if there were any there, i would only be checking to make sure everything was ok. NO drugs, , violence, etc. It's hard to trust her when she feels the need to hide this. What do you think? I am trying to be a good Mom. On our phone bill she uses about 700 texts a month. I am the one that pushed (convinced my husband)to get her the cell and then get text, and finally to get a myspace and now she has been using the myspace im and never even asked if she could. But, I was trying to be cool about that until I saw that the im history had been deleted repeatedly. What do you think??