Search Results: losing best friend

Long story short; my girlfriend of 5 years and I were at a friend's wedding. We had a lot to drink and during the walk back to the hotel nearby we got into a huge argument and "broke up". I use quotations because the next day she didn't remember (or swears she does not remember) the fight/split occurring. I, upon returning to the room the next day, learned this and rolled with it and only later told her about our argument and how I thought we had legitimately broken up that night. What I didn't tell her is how after our "break up" I drunkenly and spitefully spent the night and hooked up with an old flame/friend (a woman who was always unavailable before but equally attracted to me while I was also unavailable). This woman and I remain friends (there's no awkwardness and she's fine with the fact that my girlfriend and I "got back together"). It has now been about 6 months and marriage is in the cards very soon.
My question is; do I tell my future fiance what happened (and therefore risk losing her or causing a permanent rift)? Or do I continue to keep it a secret since it truly was at best a misunderstanding, and at worst a loophole?