My best friend made out with my crush whom she knew I liked. Should I be honest with her about how I feel about the situation?

Context:
I'm part of a college friend group that consists primarily of girls. Including me, there are 2 guys and 7 girls. The girls had been a friend group since October of last year, and the other guy and I became part of the group in February when we were all in a D&D campaign together. Everyone is super nice, and it's a pretty diverse group in terms of race, birthplace, sexual orientation, etc.

One of the girls in particular, who I'll call Kate, and I became close pretty quickly because we have very similar personalities. Around April, after a night of partying, she came back to my place and we had a long conversation. During that conversation, I told her that I had a massive crush on another girl in the group, who I'll call Ella.

Ella and I had also grown closer, and she had always reciprocated my flirting and advances. Kate was incredibly supportive about the potential relationship, and despite my wanting to keep my feelings to myself, she encouraged me to ask Ella out during basically every conversation we had. Kate also told me that the rest of the group had noticed the chemistry between Ella and me, and that everyone was basically waiting for us to start dating.

Fast forward to a couple of days ago. The friend group is still strong, and Ella and I are still in this sort of limbo where there's clearly something there, but neither of us has actually done anything about it. Kate texted me asking if I wanted to go on a walk with her, and I happily agreed.

After walking around town for a while, we ended up on a bridge, where she stopped me and told me that she and Ella had made out once in January while they were both drunk, before I was part of the group. She stressed that there was no romantic attraction between them and that they had both been intoxicated.

I said it was no problem. I wasn't even part of the group at the time, so I had no reason to be upset about something that happened before I knew either of them.

However, she then told me that just one week ago, after a party when everyone else in the group had gone home, she and Ella made out again. She again told me that there were no romantic feelings between them, but at this point, I don't know if I believe that anymore.

In the moment, I was pretty taken aback, and I think I just mumbled something about how it was fine and moved on. But I haven't been able to get the situation out of my head.

Since that conversation, I haven't seen or spoken to Kate very much. I'm conflicted because part of me feels betrayed, while another part of me feels stupid for thinking she owed me some kind of "bro code" loyalty when Ella and I aren't even dating.

I am definitely hurt by the situation, but I'm not sure what I should do. Should I just accept that this happened, chalk it up to a mistake, and move on? Or should I talk to Kate about how I actually feel about what happened?

Any advice would be greatly appreciated.