Why would a woman entertain an ex fling from 10 years ago?

Caught my girlfriend of 5 months messaging an old FWB on Instagram. This happened a few months back but I just recently found out about it.

So all I know about this guy, is they fooled around for 4 months, 10 years ago when she was in her early 20s.

When I asked the last time they talked, she said in December before she knew me and it was only a few messages. Come to find out, they had chatted in April (while we were official).

They talked about their lives now, who they are dating etc. He basically told her he wished that he didn’t ruin things with her when he had her. But one thing she told him was “you know, I thought about reaching out to you when I was single for a few months” Like why does he need to know that?

And then they kept vibing and texting with all the emojis and slight flirting etc.

So she’s been trying to console me after this. I told her it’s not the worst thing that could have happened but I still feel a bit betrayed. Which is sad because this woman checked off a lot of boxes of what you could look for in a potential wife.

She’s apologized to me many times, blocked the ex fling (tried to negotiate keeping him on Facebook which was a red flag) and then randomly says “you know what, he was the smallest and worst I ever had in bed. We only did it once” But the day I found out I remember her telling me they did it like 3 or 4 times. Which doesn’t matter but still a lie.

She keeps saying he never meant anything to her, they talked for 4 months and then he dropped her. Says he can’t be trusted and is a player. Worst she ever had in bed. So I really can’t understand why that night happened last month. I’ve asked her this question that I’m posting about, she just keeps saying I’m looking for something that isn’t there. Or keeps saying he never meant anything to her.

Recently I had her show me that she had blocked him on social media and she did, then I asked about his number and she said she only deleted it.

Currently been on no contact with her the past few days, figuring out what I need to do about this.