I shouldnt even be jealous

So i had this guy friend that i had hooked up with but like he was a cool guy and we talked shit to each other and had like talks on the phone for about 5 hours or more, we were casual he was asking for something i didnt want ( it wasnt a relationship) and we stopped talking its been like a month since then he has a gf now and i have no idea why im just so jealous, im hanging with his guy i like just seeing him with her kinda stung, and i dont know why