A Misunderstanding with Alex

I met Alex at a mutual friend's party last year. We hit it off immediately and started dating casually right after that. Nothing serious or exclusive, just spending time together and seeing where things went. It’s been about six months now, and we’ve had some great moments and some rocky patches too, but we always seem to work it out. Recently, Alex started being a bit distant. I noticed they would often cancel plans at the last minute or give me vague responses when I tried to talk about our relationship. At first, I thought maybe they were dealing with something personal, like stress from work or family issues, and didn’t want to burden me. So, I gave them space and hoped it would pass. But then one night, I was invited to a friend’s housewarming party that Alex had also been invited to. They agreed to come but never showed up. When I asked later what happened, they claimed they got stuck in traffic and didn’t want me waiting around. It seemed like an odd excuse considering the place wasn't too far from both our homes. This week, there was a mutual friend's dinner party. Alex called me earlier that day asking if it would be okay if they brought someone else along. They mentioned a new acquaintance who had recently helped them with some personal issues, but didn’t go into much detail about the specifics or this person’s background. I told them of course and thought nothing more of it until we arrived at the party. To my surprise, this 'new friend' was actually an ex-girlfriend from college that Alex dated for a few months before they met me. We've always been friends but it still felt like a bit of a betrayal to see them in such close quarters again after so long without mention. The evening turned awkward and I ended up leaving early. The next day, I confronted Alex about it. They admitted to seeing their ex more often than they mentioned over the last couple weeks while dealing with some unresolved feelings from college days resurfacing. While this isn't uncommon in relationships and they assured me everything is fine between us now, there's a lingering sense of insecurity and wondering if my doubts were justified or just overthinking.