Boyfriend problems T.T NEED GREAT ADVICE PLZ!?

so i have a boyfriend, and weve been going out for almost 3 months. Oh we went out for 8 months b4 too. so like two nights ago i was talking to him on the fone with a best friend of ours. SO my boyfriend was like oh yeah i hung out with Jamie, a friend of mine that went to my school who moved to his school, today. i was like WAT. Our best friend was like uh huh. so what did you guys do? hes like we went to starbucks and we talked and stuff. i was gettin pissed. so the next day i was feeling really crappy and pissed about that situation and i talk to the same best friend we were on th fone with. We were talking on aim. i told him everything i felt and i didnt like what my boyfriend did. My best friend was like you kno thats a semi date cuz you werent there and they literally went somewhere. i was so mad i was cryin. btw this jamie girl, She NEVER met my boyfriend until they went out that one night. she started TEXTING him way b4 she met him. She found out about him because of me, and geez now she goes to his school. She keeps telling me that she not gunna do anything but yet she keeps talking to him. i think i do have something to worry about. I msged her and i asked her so i heard you met with my boyfriend for the first time. shes like yea we walked to starbucks and yeah hes pretty cool he was teasing me about my blonde moments and stuff. I WAS LIKE UH HUH in my head.. anyways my boyfriend called me last night and i kinda yelled and said that i didnt like what he did and i dnt want him doing that stuff anymore, and that i couldnt sleep and it bothered me so much. he got pissed off.. But really i think i had a right to get mad ryt? i think its wrong for them to have done that. But what i need help with is that how can or what can i do to make them stop? stop texting and hopefully stop them from future hang outs. It's alryt for me to get furious ryt? HELP PLEASE! need great advice..