WHY IS MY BOYFRIEND SO JEALOUS?!!!!!!!!!!!!!!?

ok so my bf had a sleepover for his birthday and i didnt no. So i was in the pool and when i got dried i went to his house to hang out and stuff i live a couple of houses down from him so i came in my bikini and mini shorts i have big and i no not the best choice but anyways all his friends answered the door and theyre all like woohoo lol. So my bf came out and i said happy birthday and all this stuff 4 like 5 mins. Later it was 3 in the mornin and i woke up so i went outside for a walk and i saw them playin footy and they go HEY! wanna play too and i go ok the next day my bf is like wierd is he jelous i mean he noes i would neva cheat on him and my bf's friends respect me enough not to do crap like that. I'm popular at school and so is he an dall these girls r ova him he flirts bak but i dont react because i dont think its a big deal he's not doin anythin wrong. Y is he so jelous when i flirt with guys im not the kinda girl who cheats wat do u think. PS: he flirts with my friends 2 .