I'm a 21-year-old guy, and I have a long-distance girlfriend who's also 21. I can't shake the feeling that she might be involved with someone else.

After being apart from my girlfriend for 12 weeks, I finally visited her. On the first day—after we were intimate—I sensed that something felt off. While driving, I noticed she dimmed her phone’s brightness to send a text and then brightened it again for navigation. Although this isn't definitive proof, it reminds me of subtle signs I’ve picked up in past relationships that hinted something might be amiss. I'm going with her for the holidays to meet her family (whom I've met before) and her friends (whom I haven't met yet). How should I approach this topic?