Me [26 F] with someone I barely knew [31 M] three years ago. A weird feeling I never felt before.

I met a guy in a professional capacity almost three years ago. We worked together for 6 months, and would chat occasionally. We shared a lot of common interests, and I know this is weird, but it felt like he understood me more than anyone else has in my entire life. Even though I barely knew him beyond workplace small talk. He recommended some books and music to me that really touched me, and I still really love them now. Anytime I accidentally saw him outside of the office, I would get really embarrassed and inexplicably excited, but was too much of a coward to ever do anything more than smile.

After 6 months, he left my workplace and we haven't kept in touch at all (I was too shy to ever make a move or deepen our conversation and any texts we sent were work related). I think he might have been interested, but I;m really bad at telling if someone likes me or not. I have been in a serious relationship for two years and I truly love my partner, but I've never felt the same nervous/ warm/ pain in my chest feeling like I did with this coworker.

So I haven't talked to this coworker in 3 years. I've looked at his social media occasionally, but I don't obsess over it or look at it daily or even weekly. I'll go months without thinking about him and then BAM! I'll have a dream or thought and I just get so weirdly sad and lonely and happy and tingly all at once. I don't know how to describe it.

Surely I can't be in love with someone I barely even knew, and for years after not speaking? I know he has a longterm girlfriend from looking at his social media, but I don't really feel bad about it? I feel happy for him because she seems nice, but also somewhat lonely. He's also recently relocated to halfway across the world, so I feel comforted by the fact I won't have to see him in the same city but also really miserable about it? These feelings are very confusing for me.

I just want to understand how this can be? I feel like a creepy obsessive stalker whenever I have a dream about him or look at his social media. But its not running my life or thoughts or anything. Why do I still get this weird mix of feelings when I think about him even now?

tl;dr: Feel crazy because I still feel infatuated by someone I met years ago and haven't talked to since, even though I'm in a secure and happy relationship!