Why am I getting wrapped up in nostalgia lately?

A friend of mine that I see sometimes hang out with cane over the other day and we talked about our friends (who are now my old friends of the past) for a little bit. He even mentioned one of my old friends that I used to like with him saying we used to like each other (when it wasn’t mutual). After that, we didn’t speak of them again, but ever since that day, I’ve been like on a nostalgia attack and can’t stop thinking about it. I’ve been stalking a few of my old friends mentioned above and I just have this rush feeling inside my stomach, almost as if I’m in love? What is this?