Im feeling jelousy? ughh?

well my bestfriend and i are really close (ovbiously) and last year she had her first boyfriend & i always somehow got jelous of stupid things like me asking whtsup & she goes "nm just texting you and jake:)" (jake was the bf) and now a year later she's dating someone else who i sortof got in a fight with him but now were cool but sometimes i get mad at her cuz im jelous.......why???