Cheating?

I’m in a relationship, but lately I’ve been thinking constantly about this guy I dated for a month or so like 2 years ago. I have NO idea why though. I can’t stop thinking about him, and I have this longing feeling of just wanting to see and talk to him. I also keep thinking about if we were together and like if we would get married, just random stuff like that. It’s weird because I didn’t have big feelings for him. I feel super guilty.

Does anyone know what it means if you can’t stop thinking about someone? Should I feel guilty?

Let me add- me and my boyfriend are going through hard times and we’re not the happiest. We’re even considering breaking up. And I’m moving out.