My current gf (30F) still messages her ex but hasn’t told me

Background: So I (F 30) have been with my current gf (F 30) for close 7 months, but I know she’s the one. I bought a ring, this woman is going to be my wife, and I have never felt more sure about something. I’ve had past relationships that have never felt so secure and peaceful. I don’t feel like I’m just jumping into something, but she truly is epitome of “the when you know you know” feeling. She is my best friend, makes me want to be a better person, and our life together makes me look forward to growing older, and enjoying our lives together.

We get on each other’s phones on and off for things like playing our mobile games, checking random tid bits the other asks, changing music, etc. There’s never been secrets or shyness around the other person seeing what we’re doing because I have nothing to hide, and I assume, neither does she.

One of the times I was checking our games I accidentally swiped too far up and went to the open apps screen. On one of her social media messaging apps, one of her exes had messaged her recently. This is an ex we have talked extensively about in the past where things were not good between them when they were together, and she told me that she hated him. I had asked if they still talk, she said no. Fast forward to the moment I saw the message on her phone: I was shocked, hurt, and also confused. Part of me wanted to think nothing of it, but there is a part of me that also feels uneasy ( I read the messages because an insecure part of my brain told me to because my prior relationship ended when I found out my ex partner was cheating on me which I saw through messages on their phone). A lot of it just seemed like small talk, comments on different videos, At one point in the conversation, one of them had mentioned right person, wrong time. That’s when I spiraled. We’ve talked about marriage, about this being it for both of us. That was hard to see.

I know she’d ever act on anything or go see him. I don’t understand why she wouldn’t be honest about the fact they still talk, even if it’s they’ve found a way to be friendly.

The confusing part is really why wouldn’t she say anything to me when we’ve talked about our exes and situations from the past. It’s the only hesitation I have. I’ve considered bringing it up, I don’t want her to get defensive or shut down with me during the conversation.

What do I do? How can I approach having the conversation?

Please help 🙏