My (25m) gf (24f) spent the night at her ex's house. How do I proceed?

I've lost my words so I made this post on a throwaway account for obvious reasons. I won't get too deep into the story, but I need to know how to proceed with this as I've literally lost my words. I'm in a 4 year relationship with my gf. We see each other every week or every two weeks, sometimes three but that is rare. We don't live in the same city due to work. She went to her family for a few days and last night she drove back to her apartment but didn't spend the night over there. She woke me up with text saying 'I'm with my sister'. This is a code phrase she uses when she is actually with her ex. I'm ok with this because they have a kid. She's free to see her ex. I understand they have a child. I fell asleep and the next morning I got another text saying she's back at her apartment. I asked her how she got back to her apartment, and she replied that her brother gave her a ride back. Then I remembered that she has no car key on her, because she keeps it at her apartment. When I asked about this, she said 'it doesn't matter'. I tried to convince her to tell me the truth and she said 'I don't want to argue with you over nothing'. She is 3 hours away from me, so it's not a pleasant drive to go there and talk about this. I'm planning to see her this weekend, but I don't know how to act and what to do during our talk.<br><br>&#x200B;<br><br>Honestly, the trust is broken. I can't live like this again. I was cheated on before. I know what it feels like. And I'm having the same feeling again right now. I don't want to accuse her though, as there is no concrete evidence other than the fact that her story doesn't make much sense. But I don't know what else to think. I asked her to tell me the truth, and she didn't. So either she's lying or not. She has been throwing the 'don't I deserve the benefit of the doubt' phrase lately. I'm really lost and hurt rn. I don't know what else to do.