Am I unreasonable for feeling weird about his girl bsf?

So my boyfriend and I have been dating for 1.5 years. We're in college, which is where we met, but he transferred after our freshman year and we kept dating. I see him every other weekend.

He had trouble adjusting & making friends, and I feel bad about how he was treated by the people he befriended in the first few months after transferring. Luckily, he joined a frat and is doing better.

What's making me feel weird is the presence of this one new friend of his. They met at a mixer and hit it off because she's the ex-gf of one of my boyfriend's former bad friends. Since then, they've gotten super close, which I was initially really happy about!

But lately, it's felt weird. Bc we're a LDR, I can't always be there for the events his frat has, so he'd have to go without a date. But that's not happening, because she's lowkey becoming his go-to fill in for me... In fact, he's taking her to his formal tmr, even though I offered to be there. He said he had too much work for a visit.

I've noticed him hiding his phone screen from me when we're together while he's texting her, and acts really strangely around her; for example, if we're hanging out and she's nearby, he'll constantly look over at her and start to ignore me. Last night, he hung out with her until 5am.

I'm not usually insecure about female friends, bc my boyfriend mostly has female friends. When I call him out, he has reasons - he's really anxious so he's nervous around new friends, he needs someone to take to his frat events or he'll be alone, etc. - but I still feel off.

He's said she's like his "big sister" and that I "obviously shouldn't worry" because I've met her and they're just friends. But around when they started getting close, I noticed him pull away (which he admitted to doing but said wasn't related). So I'm starting to become a little uncomfortable, all things considered.

Should I wait until a firm boundary is crossed to say smthn? How do I bring this up without coming off as insecure?

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Well, anyways. He WAS cheating.
Am I unreasonable for feeling weird about his girl bsf?
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