Relationship advice

My fiance and I have been together for 5 years. We have 3 kids together, 4, 2 and 1 week old.

In April 2022, I was 6 months pregnant and basically from January this year up until June, we were not getting along. Everyday was an argument, we had some changes with his daughter coming into our care, financial changes for the worst, so there were reasons to cause strain.

One night, my mum had our 2 youngest and his daughter was with her mum for holidays, he worked late as it was Easter weekend and he works in hospitality. I thought I'd cook a nice dinner, get his favourite snacks and when he got home (9pm) we would have a movie night and just be kid free and relax.
Well, all went well until bed time came and he asked which room is he sleeping in, because the night before we argued and he slept in the spare room. I shrugged and said wherever you want even though I really wanted him to stay in our bed. He chose the spare room and that sparked emotions for me. I cried. Then I texted him and said it made me feel sad. He ignored my message. So I walked into the spare room where he was and he was scrolling on facebook. That pissed me off more. It turned into an argument and then he flipped it. He was screaming out hurtful things, before he walked out the door he said he couldn't give a fuck if I leave him anymore then got in his car and left.

I had his messenger account on my phone also, which he knows and within 10 mins of leaving our home and me crying, he had messaged a young girl he works with, 19, and asked if she is out. She replied and said yes are you? He said Na I'm just driving around. She then asked for a lift to get food and then home. He said yes and picked her up. I called and called and called and he didn't answer. I messaged and said who is in your car just to see if he would be truthful and he replied and told me who he was with and that he was just dropping her home. I called again, he didn't answer. I then called her on messenger and she didn't answer. He called me about 30 mins after he picked her up and that was when she wasn't in the car anymore.

I told him how hurt that made me and asked what they spoke about, why he messaged her out of everyone and I just had a billion questions. He said she was active online so he just asked and there was nothing more to it (I still can't settle for that answe) - he said they spoke about work and he did speak about our relationship, that we have been arguing alot and that I also am a good person, a great mum etc. She apparently said she hopes we can work things out and that her being in his car isn't going to make things worst.

Weeks past and it still comes back to me and I ask him again, did you cheat on me physically that night or have the intention to? He says no. He says he would never physically cheat on me because I don't deserve that and he wouldn't do that to me. But there's still something inside of me that's telling me otherwise.

I've thought to message the co worker, but I can't bring myself to do it because she's young and reality is, wasn't really her fault being dragged into our argument.

I have been to his work a few times and she has been there, I found it awkward and couldn't say hi, he asked me why I can't say hi and that it doesn't need to be awkward? He invited me to his work for coffees even while she is there and that sort of helps put me at ease that he doesn't make me avoid her....

Anyway, 3 months since this has happened and recently we have been getting along alot better, finances are better and stresses that we had aren't there anymkre, he has been making an effort with me, emotionally, listening when I ask questions, responding and putting in effort of communicating with me but it's just that one thing still ticking on my mind of, why did he reach out to her that night ?