I completely agree with everyone’s ideas. You need to sit her down and have a serious talk with her as soon as possible and nip this in the bud. If you don’t think you will be seeing her within a week, then set up an “appointment” to meet with her. Don’t let her know what it’s about or you will just have an ugly “she said, she said” conversation on the phone. You owe it to yourself to have this conversation face to face.
Try to remember the exact date that you loaned her the phone and then say something like, “Erica, the reason I wanted to talk to you in person is because something has been really bothering me lately. Remember last Tuesday when I let you borrow my phone to make that call when we were at Robert’s party? Well you didn’t notice, but I could see you and I could see that you were scrolling through my private text messages and ever since that day, you’ve been making little sarcastic jokey comments to me, based on the information that you found in my phone and I’m extremely upset about that.”
At this point, she is likely to get indignant and start accusing you of being a bitch and making up stuff. Try not to let it get to you, try to control your own emotions and not scream back at her or start crying (there’s plenty of time for that after she leaves). Simply repeat that you know that she looked at the texts because whenever they’re viewed, it leaves a record and you have checked that your texts were indeed looked at that night. Plus you saw her scrolling through them. And she’s been teasing you with private information that she could not have known otherwise (even if she could have, it’s not relevant in this case).
Even if she tries to apologize, which she probably won’t, just let her know that you don’t feel like you can trust her anymore.
If you really want to have some fun, talk to another friend who also knows this girl and let her know what happened, then ask her to send you a text before you have the talk that says something like, “What a bummer that Erica went through your texts. I can’t believe anyone would actually do that. I won’t be letting her use my phone anytime soon.” Then say to Erica, “By the way, you missed one of the texts care to read it?” Either she’ll read it herself or you can read it aloud to her. Either way, she’ll get the message and she’ll know that she’s been ratted out.
Then just limit your involvement with her.