My boyfriend and I have been dating for 3 years now. When we were in 11th grade he made friends with a quiet, awkward chick (Rosemary) and since then she's joined our "friend circle" I guess you could say. We recently just graduated but this whole year had been an upside-down trainwreck for me. He started becoming really close with her and she went to New York for a while and when she came back my boyfriend said something I had not forgotten ever since, "Things were so boring without you around."
Skipping ahead a few months, my boyfriend recently got a new cellular device (we don't care for them but his mom insisted to get him one). He gives his number to her first and the first thing Rosemary says is "I'd blow up your phone so much!". That left me a little confused and worried. I trust my boyfriend but I do not trust or feel comfortable with her and I've already let him know that. But I think my trying to get him to understand that she likes him has pushed him away.
He texts her often and sends snapchats to her, I got a little paranoid and looked through his Facebook messages to find out she and him call each other "Partner In Crime". I felt disappointed, left out, abandoned and afraid. He was upset with me, but I had to know. She's good friends with his sister and is quiet/awkward around me. She tries to befriend me but I already know how that works out in the end.
I just need help, am I overreacting? Is there something I should do? I haven't confronted her, my boyfriend doesn't want me to. Is that a sign that something is happening behind my back? Please help!