Any bi women that used to identify as a lesbian?

Hello, I’ve been looking for some insight into this but haven’t really found any yet. Are there any bi women out there who used to identify as a lesbian or thought they were before coming out as bi?

I ask this because I am having a bit of a weird reaction to my really good friend going from being super open about being a lesbian, to her coming out with “I think I like guys” and getting into a committed relationship with the first guy she matched with on some dating app. For context, she was my very first girlfriend back in high school. She identified as a lesbian, and I always just knew I was bi. Our relationship sort of fell apart after she decided to start exploring bisexuality with another guy at school while we were dating and I became really insecure about it. We break up, talk about it, and a lot of time passes but we’re back to being good friends. She also went back to identifying as a lesbian. I thought I was over it, but recently she’s been so…talkative (?) about her new relationship. Which she also just never was with any woman she ever dated. And it’s been making me feel like an idiot for not being fully happy for her. A part of me gets upset every time she brings it up.

I know sexuality is fluid, and people change and all that. But I wonder if anyone might be able to share some insight on what it was like to go from identifying as a lesbian to bisexual? I’m hoping that maybe if I can understand her and her struggles, I’d have an easier time being mature about the whole thing. I just want to be a supportive friend!