My (m) girlfriends (f) camping trip. What do you think really happened?

My wife S and I have been together for over 20 years. We got a couple in highschool and stayed together forever. We have had our ups and downs but never had a real crisis that would threaten our relationship.
I've never cheated on her, and never had the urge to do so. She says she never has cheated on me and I believe her. Except for one story, dated more than 15 years ago, that I would like to call her camping secret.

Every year she goes on a little trip with her best friend V. They were still students both 20 years old, so they had a limited budget and decided to go for a camping trip in the Ardennes, a beautiful region in Belgium with many woods, little rivers and nice hikes. They choose a little campingplace, far from the comercial big campings with all those playgrounds. It was the time before smartphones and fast mobile internet, so the only contact we had was a text message at night, where she told what they had been doing: kayaking, mountainbiking, hiking, … and so on. The last evening however, I got the usual message with their activities, casualy followed by the fact that they had met some guys around their age and they wanted to go for a swim at night in a nearby lake. It would be fun for their last evening.
I was quite jealous at that time, but didn't say anything about it. She often said I was to suffocating in our relationship and I didn't want to fight in textmessages. Besides, I was sure i couldn't convince her not to go, so why doing effort?
The next day they came back home and i picked her up at the trainstation. After the first warm hugs and kisses I asked her about their swimming adventure. She gets quite cranky and says 'There is nothing to say about it, we just went for a swim and had some beers and wine. That it, no reason to be jealous about it'. I startled a bit from her raw reaction and decided not to go further into it and i forget about it.

Years later, we just moved in together, we had a bbq: S, me and her best friend V. There was lots of food, lots of drinks and lots of fun, we were all having a good time. The girls started brining up memories about all the trips they had done togetjer: London, Paris, Amsterdam, Malta. And then, all of a sudden, V shouts laughing: Do you remember our camping trip? That last night when we started with skinny dipping with those guys and than ….' Than she realises it's not just her and my wife S, but that I'm there also.
I got overwhelmed by a feeling of jealousy and hornyness. I could only say 'you never told me you went skinny dipping. You said there was nothing to tell about that swimmingtrip'. S gets all red and tries to save her situation: 'No, it was only V who got naked, I kept my bikini on'. V: Not at all, S. It was your idea. You even didn't want to get your bikini on because it was still wet from kayaking and you even said that was a very good excuse to go naked. But okay, now i'll stop about it, before the real trouble start'

In meantime, I got so mad that I ran away. When I came back, hours later, V was returned home and S was sleeping. Every time I start about this event, they both got mad and refuse to have a decent conversation about it. 'We went skinny dipping. So what?'

I'm really dieing to know what really happened that night. What do you think?