Wife's Secret Office Hook-Up

So, yeah, my fiancée and I are engaged. Been together for two years, planning the wedding all that jazz. But last week, she confessed something to me at dinner. Guess while I was out of town on a work trip, she had this thing with one of her colleagues in the office. She's a teacher, he's an admin guy about my age who used to flirt with her before we got together. So, long story short, they hooked up once when she came into his office at night after hours. It was just that one time and she's like, 'I don't know why I did it' or something. Anyway, now here we are. She says she wants to make it right with me, that this doesn't change how she feels about us getting married. But honestly? I'm feeling pretty damn conflicted here. So, am I being ridiculous if I still feel like I need more time before tying the knot? Or is there a chance this could all be smoothed over somehow?