NEED ADVICE!! Am I overreacting?

So my fiancé and I have been together since June 2019, he proposed in November later that year and everything has been good. We just moved in together last month but haven’t been wedding planning because of COVID and all of the uncertainty on when this will end. Saturday night he got off of work and parked outside of our apartment, I saw his lights go by so I looked out and saw he was outside, so I got up and unlocked the door. Just as I was doing that, he text me and said “I’m on my way home.” I thought he was joking so I played along and was like okay, cause he’s done stuff like that before where he said that but then popped up at the door two minutes later and made some kind of corny joke. But this time he sat outside for another 10 minutes. So I text him and said aren’t you outside, he said no. So I called him, mind you I’m still looking out of the window, just as I started calling, he starts the car and pulls off. I’m like 🤔 Uh okay. But he said I ruined his plan of popping up. I let it go. But something told me something was up, so this morning he left his Apple watch at home, he doesn’t know that even if he deletes a thread on his phone, his watch still has the thread in there unless he goes and deletes it out of the Apple (tip in case y’all didn’t know that...the watch tells everything lol). So I open it and I see a thread from Sunday morning with some lady named Tiana. She said “I wish you would call me back I know we just met but I don’t mind if you have a gf...can’t stop thinking about you.” The thread started Saturday night at around 9 where he text her and asked how she knew he was looking at her. She asked if he could meet up and he said no but you can call me. This was at 10:54pm, in that same minute that’s when he text me and said he was on his way home. But the very next text he sent was from 4am Sunday morning and he said that he made a mistake getting her number and he didn’t know why he did it, he didn’t wanna hurt me, he knows he wants to be with me, and asked her not to respond. Now my question is, even though they only sent 7 messages back and forth and he came to his senses pretty quickly, am I wrong for still being upset. I mean he didn’t tell me about it, he lied about leaving work, the real reason he was sitting outside was because he was talking to her, etc. My friends keep telling me if he ended it quickly it’s okay, but I feel like that’s accepting disrespect. He shouldn’t have gotten her number and flirted with her at all. And I shouldn’t be expected to just let it go because nothing happened, what if next time it goes a little further, so on and so forth?? Am I wrong??