Search Results: my ex girlfriend just got cheated on

I asked a question about something similar to this before.

A while ago she told me her and her best friend used to have history and have “fucked around before” (she said didn’t they didn’t have sex but he’s grabbed her boobs and I’m assuming made out or whatever)

and at first I was a little nervous that she’s best friends with a dude she’s done shit with before. But I trust her, and I decided I was being a little insecure due to being cheated on in my last relationship. So I forgot about it.

Today I was walking home with her and we were going back and forth with the little roasts/jokes we do. (We’re both sarcastic people, we’ll poke fun at each other sometimes)
And one it her responses to me was something along the lines of “Okay, I’ll kiss my best friend in front of you”

she was joking around but it was just a joke that was too real for me considering I already had concerns about her being attracted him..

And it was very out of nowhere. I don’t remember what I said but I remember not understanding the correlation of why she would bring him up.

I didn’t get mad at her or anything because I felt it was just something stupid I was overthinking about.
I just get nervous because my ex used to make “jokes” about having a second boyfriend and it turned out to be true...
So am I ignoring red flags or just being insecure?