If you're worried that she's cheating, look for all of the signs in this article, such as being secretive with her phone, changing her appearance, being less affectionate toward you, and disappearing for periods of time. However, keep in mind that it's normal for people to have friends at work and maintain friendships outside of their relationship. It's best to talk to your girlfriend about why you're concerned about this particular coworker. Then, decide if you're able to trust her or not.