Borrowing a friend's login to post this

So it started last weekend. At first I thought it was just a coincidence, but small things kept piling up and now I'm doubting myself more and more. I'm 26, a teacher at a small school in Nonthaburi, living with my girlfriend — we've been together almost two years. Overall things were fine, we fought sometimes like normal, but I never thought it'd get to the point where I'd be this suspicious.

Short version: I'm so confused.

What happened was on Saturday she said she'd go help with something at a relative's place near Muang Nonthaburi in the afternoon until evening and then she'd come see me. I had tutoring that night from around 6:30–8:00, so I didn't think much and waited for her texts. I looked at the analog wall clock and it said about 9:40 PM. I thought she'd come back briefly to drop me off before my class, but time kept passing — by past 9 she wasn't answering calls, she read messages but didn't reply. I called again and again, worried. Finally she called back at 10:15 PM. Her voice sounded tired, but not the same kind of tired I'd heard before. There was something off that I noticed, but I wasn't sure if I was overthinking.

The next day I randomly saw an Instagram story she posted — a backlit night shot of two people standing, just silhouettes and light shirts. No tags, no clear caption. The shadow of the person standing close to her made me feel weird; it wasn't the usual group of friends I've seen in photos before. I tried not to jump to conclusions but my heart started racing. Then I noticed there'd been messages from someone in her phone over the past month with a name I didn't recognize — sometimes short jokey texts or things like "wanna meet?" I never asked about this person directly because I was afraid of digging up distrust too soon, but the doubt ate away at me.

I used to be someone who trusted easily, but that's changed. Some nights I even grabbed her phone to check how this guy talked to her, then stopped because it's a huge invasion of privacy. But the suspicion keeps looping in my head. I feel guilty for thinking the worst and sad that our easygoing relationship has become like this.

Maybe I'm just overthinking. Or maybe I'm actually jealous, I don't know.

There are other small things too: she started turning her phone off-face when she's with me, or putting it face down. Tiny stuff, but for someone who used to read out her phone stuff to me and make me laugh, it's a big difference. The little details used to be everywhere — she'd tell me things like "something funny happened in the teachers' room" or "I ran into an old friend" — now she tells me much less. When I ask she answers short: "tired" or "nothing." I try to be understanding and give explanations the benefit of the doubt, but it still nags at me.

Some coworkers have started noticing because I'm usually chatty, but lately I've been quiet — so quiet that a senior teacher asked if I'm okay. I don't want to blow things up at work, so I only say I'm stressed about personal stuff without giving details. But inside it's burning.

I can't sleep some nights thinking about that other guy. If it's true, what do I do? Confront her now or gather proof first? Wait until it's clearer or trust my gut? I hate the uncertainty. If I confront her directly she might say I don't trust her or that I'm making things bigger. I'm afraid I'll destroy what we've built, but I'm also afraid of living with not knowing.

I'm planning to have a serious talk, but I worry that if I'm just being paranoid I'll hurt her and make her resentful. Sigh. I'm so confused and tired.

Anyone who's been through this, please give advice. Should I confront her and go straight to the point, or wait for solid proof? Or should I trust and let it go because maybe I'm overthinking? I'm scared of making the wrong call both ways. Please help, what should I do?

P.S. Sorry for the long post, I'm venting so much. Seriously, so confused lol haha