I'm a 23-year-old guy, and my girlfriend, who is 22, often spends time with a coworker who has expressed interest in her romantically. How do you interpret this situation?

I (23M) met this girl (22F) about eight months ago on my university campus in the States. We hit it off right away and went on several dates, officially becoming a couple just four months ago. The relationship has been wonderful; she’s an amazing person who I believe truly loves me, and what we share feels special. Recently, she started her first job in the corporate world. Unsure of how things would unfold, she mentioned she didn’t want to share details about me with her colleagues for the first few weeks to gauge the workplace dynamics and see if people generally talked about their partners. I understood her reasoning and was okay with it. However, a few days into her job, a coworker (22M) began reaching out to her frequently, texting and calling her often, even setting up Zoom calls for work prep. My girlfriend expressed that she enjoyed their conversations and thought he was a cool guy. They shared long, deep chats about past relationships, crushes, and life experiences, which made me feel uneasy, as it seemed like he was trying to flirt with her. What struck me as even stranger was that she hadn’t mentioned me to him at all. When I brought up my concerns, she was receptive and agreed to tell her coworker about me. A few days later, she did share that she was in a relationship. Despite this, later that evening, the coworker called her and confessed his feelings, asking her out on a date. My girlfriend clearly told him she wasn’t interested and that she liked me. However, after that conversation, they still spoke for about 30 minutes, during which he attempted to flirt with her again. I didn’t think too much of it at the time. A few days later, she started going to the office on days when she could have worked from home, inviting this coworker and another coworker (22M) to join her. They began hanging out regularly, spending time together in the office when others were working remotely, and going out for team lunches and other outings. While she assured me that they maintained a strict friendship, there were instances of flirting from him. He even offered to teach her how to ride a bike, which really bothered me since I had always wanted to be the one to do that. It made me question whether this was normal and whether I should be comfortable with her spending so much time with someone who clearly seems to have feelings for her. What do you all think about this situation? How should I approach this in my relationship? I genuinely love her, and I know she loves me too, but this whole scenario feels a bit off. I’d appreciate your thoughts on the best way to move forward.