Am I overreacting by suspecting my partner might be cheating, or is it just my insecurity?

Hey Reddit, I'm a 30-year-old woman seeking some perspective because I feel like I'm starting to spiral. My significant other, who's 27, has a close coworker, and even though he says she's a lesbian (which I completely respect), their relationship makes me uneasy. They communicate constantly, often more than he and I do, and she frequently joins his Twitch streams, even when we're gaming together. Whenever I bring it up, he downplays my concerns, saying I'm overthinking and reminding me that she's not interested in men. Still, my instinct tells me that something isn't quite right, even though I recognize that my feelings might be influenced by insecurity and past trauma from relationships that ended due to "just friends." I've been honest with him about my insecurities and history, but it feels like he's dismissive of my feelings. I don’t want to project my issues onto him, but I also don’t want to overlook any potential red flags. So, Reddit, am I overreacting? Is this feeling rooted in my trauma and insecurity, or is there something that warrants a conversation? How can I address this without coming across as accusatory or irrational? Thank you for your support.