Why do i feel jealous of my boyfriends friends who are girls?

me and my guy live together and we both work alot and nights, but he works weekends and i dont. so on the weekends i go out wit my friends which is one guy but hes gay. but now my boyfriend started talking to his old friends from elemerty school and its a girl, but this girl was dating his bestfriend all through high school but now they broke up, and she is talking to another guy but then she told my guy the other day that back in the day she thought he was really cute. but i trust him i really do and i love him a lot but i just get jealous cause his weekend is mon-tues he has those days off. but im kinda scared cause he talks to her on the phone alot while he is at work and i just dont want him to start talking to her and liking her, i mean right now as i type this he said that she is over there and that another friend is going over to but a guy. i dont know what to do i mean she knows me and supposely she said she wanted to hang out wit me and him but why go over when i left for work