Is this a cheating red flag?

So I’ve been with my boyfriend for about four months and I really do love him and I have never had a connection like this before. But he has a female best friend that’s really pretty (Hannah) and I never was concerned because I think it’s healthy to have friends of the opposite sex. She knows about me and she even helped him plan my first date with him. Now I wasn’t concerned about her because I figured if there was a spark then something would’ve happened between them.

But about 2-3 weeks ago he slipped and told me that him and her went on one or two dates a few years ago since they were such good friends. I know that I have an issue with trusting people and my last three boyfriends all left me for their exes (really bad luck on my part tbh) but it was really painful for me and that’s just a fear I will always hold on to. And I also have a guy best friend Logan that my bf was worried about but Logan is literally gay so my bf’s fears on that really disappeared.

But my issue was the fact that there’s an undeniable mutual attraction between the two. And when my bf asked me if I was mad at him for having gone on a date w her I said no (even tho I am I know I can’t change the past) but I told him that I’m mad he kept it from me because that only makes me wonder what he’s hiding.

Since expressing that fear I saw that he’d text her late at night (she has a bf too) once he’s told me goodnight. And sometimes she replies sometimes she doesn’t. But also since telling me he’s been trying to position his screen away from me when he’s sending her snapchats. And the snaps are either of me or something really generic. But he would never hide their friendship until that moment.

I find it weird that he texts her after midnight and I only know this cause I snooped his phone. The reason why it’s an issue to me is because he tells me he’s going to bed at like 11? Knowing that I don’t sleep till 2 or 3 in the am. I’d be mad at this if it was his guy friends too but it makes me even more upset that it’s Hannah.

Also this one day him and I were laying next to each other on our phones and Hannah was texting him about her New Years. So she literally sent him a video of her drunk and dancing and playing w her boobs (she was at home pregaming) it was like not even 20 sec long. But i found it really nasty seeing as she was sober when she sent him this the next day. So he just tells me that she’s never done that before. And I didn’t believe him tbh

But I’m more worried about confronting him bc I did already share that I found it weird he didn’t tell me about how they once tried to date and after that he’s been keeping her as more of a secret.

What do i do? Am I overreacting?

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But I’m also worried that since he’s her girl best friend he’s going to go to her whenever we argue and she’s going to find a solution or take his side and whatever. And he’s going to always have a written account of all the times we argue (Bc you can’t jut share the good times, every time with everyone lol. Good times don’t warrant advice) and he’s going to assume that Bc she sees his side and that she’s his escape for when we argue that he’s going to want to be with her instead of me. I think it’s a pretty insecure thought of mine but I also think it’s kinda valid