Why does my Friend Treat me like ****!?

Okay, so im 13, I have been friends with this boy for about a year and a half now, we never really used to be very popular so we got close and like hung out like evryday and be became bestmates ect. now im in year 8 we both started hanging aorund with this girl who was quite popular and then we got close to her and then her more popular mates ect.. (we wernt just using people to try and get popular) now we are really close with a girl in our year and she is probs the mst popular therefore we are quite popular. Back the the boy i was origanily close to, whenever we are hanging around with her he always calls me a tramp, laughs at me, makes up lies, talks about me when im not there. Yesterday we were playing out and going to meet up with her later in the day and he was fine when it was just us two, ie being nice, having a laugh but as soon as he is with her he starts being a towards me, what should i do? its really pissing me off! x