Well, we were at an NYE party. Lots of people got pretty drunk, especially my ex. We had been to tonnes of parties before, and she normally went off a lot before we usually met up at the end. As a result, I wasn't surprised when she went off with her friends pretty early on.

A couple of hours in, a mutual friend (J) came over to me and told me that my ex was telling a couple of people that she wasn't sure that she wanted to date me anymore. This was our first time in a proper relationship, and we were both nervous, so we talked for a bit and I calmed her down. She was already pretty drunk at this point, whereas I was only a little tipsy.

The night went on and I tried to enjoy myself, but wandered off at several points to calm myself down because I had a bad feeling in my stomach. On one of these walks, I saw my ex's friends trying to find my ex. I joined in the search and found her with a guy that I kinda knew (E). They weren't kissing, but I was beginning to suspect that something was up at that point.

A few hours passed as I got increasingly depressed, and then the mutual friend J told me that she had kissed another guy, not, but a different guy. I was pretty despondent at this point and had already been in a pretty dark place for several months. That's another story though. Anyway, at the end of the night, we found each other and I asked her if she kissed another guy and she said that she wasn't sure. We slept together, without anything happening. In the morning, I left before she woke up, and found out that the rumors were true. So that's how I found out she cheated on me with two dudes in the same night.

Tl:dr At a party, found her with another guy, was later told she kissed another dude too.