#boyfriend issue

Okay, me and my boyfriend have been together for 7 months now. I'm his first girlfriend, and everything else. And he doesn't talk about girls that he's talked about in the past much to me. Which I understand. But I occasionally find him texting a girl who I am guessing it is a former girl who he has "talked" to. She lives in Florida, but hours away. But it still bothers me seeing them text or communicate. It's been going on for I think 3 months into our relationship and I still haven't said anything to h about it. I'm not going to be the bitchy girlfriend and say "you can't talk to her!" Because that's unfair to their friendship. But then again I don't even know about their friendship, or who she is! He knows all of my guy friends, and if he ask how I know him, I'll tell him the truth. But I have no earthly clue who she is and if I should feel threatened by her. I am quite jealous of the fact of how pretty she is, but then again, it's a girl who my boyfriend used to talk to so of course is bothers me. I feel like not talking to him about it will make everything okay in our relationship. But it truly does bother me. I've been told to just let it go because I am the one he chose and the one he wants to be with. But what if she pops up out of no where? Should I be prepared? Should I converse with him to really know who she is and where she stands with him. I don't want to be the bitchy girlfriend, but I do not want to be caught off guard. This really fucking sucks.