I Trust Her, But This One Thing Keep Stuck In My Head

I've been seeing this woman for few months now, and so far I really can't complain. She always honest with me, never hide where she going, and we have good time every time we meet.

Few days ago she told me she was having dinner with some friends after work. Later she send me one group picture, and I notice one guy she told me before was her ex. She never try hide it. In fact, she already told me early on they still in the same friend group and sometimes see each other.

The problem is not what she did. The problem is my own head. Every time she mention hanging out with that group, I start wondering if I'm supposed to feel okay with it. I don't want ask her stop seeing her friends because that don't feel fair at all.

I also know if I keep acting like nothing bothering me, maybe one day it just come out in wrong way. I rather talk about it now if that's the better choice.

How would you bring this up without sounding like you don't trust the person? Or is this something I should just learn to get over by myself?