She's probably upset that you're asking her about who she's talking to. While you might feel worried, she's allowed to talk to people without telling you about it. It's important that she have some privacy. If you think she's cheating, it's best to talk to her about your concerns. If you notice that she's secretive with her phone and shows other signs of cheating, tell her that you've noticed these behaviors. Then, decide if you can still trust her or not. If there's no trust, it will be hard to save your relationship.