How do I tell the girl (22F) I’m (24M) dating about my mistake

Hi guys

So something happen a few days ago that has made me feel guilty. I (24M) have been seeing this girl (22F) for about 8 months now and we are dating exclusively. It has been a rocky relationship but we do love each other deeply. I had asked if she wanted to be my gf and she declined due to other reasons so technically we are not official

A few days ago I was on an app called Wakie where you can talk to random strangers about hobbies, politics, whatever. I was on it for a bit talking to both men and women about specific interests, like film history and such. I took a call from a women (didn’t even have a pfp) and for about 6 minutes it was pretty normal, just talking about tv shows. At around minute 7 she started to become flirty, calling me things like “good boy” (I know, cringe). I was kinda shocked but also didn’t end the call right away. I said things like “why’d you call me that” or “my voice changes when I get nervous” and stupid things like that, playing along. This lasted for maybe 1-2 minutes and then I hung up, realizing that it wasn’t right for me to be doing that. I blocked her and deleted the app and everything, but honestly I still feel guilty for even entertaining the flirt.

I understand people have different standards for what they consider cheating and all, but me and the girl I’ve been dating haven’t gone into those things in depth. Despite that, it definitely felt wrong and I feel awful. I’ve barely eaten since it happened. I think I’m just going to be up front with her and tell her what happened so she knows. I suppose my question is mainly, how serious of a transgression is this?

TLDR: I entertained a very brief flirtation and feel guilty since I am exclusively dating a girl