I’ve been working with this guy for the past eight months now. We are both student workers at a restaurant on our college campus. He's really friendly, but also kind of nerdy. There was one time when he asked me what I was doing that weekend (we asked each other that question frequently). I told him, and asked him what he was going to do. He then said that he was going to visit his friend at work. He said "I'm not going to say no to bothering someone at work. I already bother you enough. Or maybe I should bother you more?" I didn't know he was flirting then, but I do now.

Two weeks later, I invited him to my birthday party. I figured what better way to show him that I'm interested than by inviting him? I messaged him on Instagram. He messaged me back saying he wasn't sure, but that he'd let me know in a couple of days. That week at work, he was acting kind of distant. He wasn't necessarily cold, just not quite as receptive as he had been before. He told me later that he wasn't able to make it. For the next couple of weeks, he was distant. I never knew what I was going to get with him. Some days, he was talkative like always, and other days, he was avoidant. Keep in mind, the same week that I invited him to my birthday party, he accepted a promotion at work, so he has the title of department lead. Department lead is basically like assistant manager.

After about three weeks of the behavior, he's been somewhat back to normal. Because of the new semester, our schedules don't match up as much anymore, so we see each other less often at work. However, he's been friendly. One day, I said hi to him, he said hi back, and I had to go sanitize tables. When I came back, he gave me a hi five and asked me how I'm doing. It was Valentine's Day, and it could be a coincidence, but he's never done that before. We usually just say hi and that's it. In fact, he doesn't like touching people in general. At least, I've never seen him voluntarily touch any other coworkers. Just a few weeks ago, he came in, and asked me if things had been busy. I told him it was slow. So then he said "so you've just been hanging out looking pretty the whole time?"

Tl;dr: guy I work with showed a lot of interest in me. I liked him, and invited him to my birthday party. That same week, he also got promoted to assistant manager at our work. He started being really distant, and now, things are getting better