My Long Distance Relationship Ended Because of a Work Event

I just got off the phone with my ex-girlfriend after what was supposed to be our first date in months. We’ve been long-distance for six months now, but it seemed like we were finally getting close to being able to see each other more often. She mentioned planning a surprise trip to visit me next month. But then she brought up the work dinner party I attended last week without her knowledge or consent. It was an event organized by my colleagues where everyone had a good time, and it’s not like anything untoward happened, no flirting or inappropriate behavior from anyone, as far as I know. However, there was this moment when one of my female coworkers leaned in to whisper something humorous to me, and at that instant, we sort of shared an intimate look. It felt almost conspiratorial. I can’t tell if she misinterpreted the situation or is just insecure about our long-distance relationship. She accused me of leading her on with false promises and being dishonest about my social life while apart from her. The conversation became strained quickly after that, but I couldn’t deny feeling some guilt over not informing her beforehand. I’ve always been truthful with her, and this felt like a breach of trust despite everything being innocent. Should I have told her about the dinner party? Was it wrong to keep something as seemingly innocuous as this from her? Am I really the asshole for attending an office social event alone when we’re supposed to be long-distance dating? What should I do now that she’s upset and I don’t know how to mend things?