Why am I getting jealous after rejecting him?

Anonymous
I rejected this guy but I have been interested in him for a while but he never seemed to approach me. I then ignored him for a while just to see what he would do and get my head straight and focus on other friends. I'd also be quite shy around him so we don't really talk a lot (my fault as well) but he contacted me via text (we talked for a bit there just small talk stuff) then he asked me whether I wanted to hangout. I declined and said asked him whether he liked me or something and he responded with a yes and was beginning or like me. I think I must've panicked but I told him that I didn't see him that way and that my 'actions' of checking him out and stuff were just me trying to be friendly. He politely said that he must've been mistaken and I apologised. I started ghosting him for a while and just viewed his Instagram stories and Snaps but then he created a new account where I can't see anything anymore. I saw him walk with a girl home and just had this feeling of jealousy. Sometimes I just can't keep my eyes off him how ever hard I try, my eyes would just go his direction and if we ever looked at each other I would look away. My best friend also took a story of him and posted on her Insta story and I got so jealous. Why am I getting jealous? And do you think what he did was wrong?
Why am I getting jealous after rejecting him?
3 Opinion