So my boyfriend and I broke up for reasons that had nothing to do with our relationship really (we were happy, etc). It was awful for both of us. We got back together after 6 months. But during that time one of his housemates was this girl from abroad. They are really good friends and generally I have no issues with him having female friends. He's friends with an ex that he was very serious about and I honestly do not care. But this girl (who is now back in her home country so I have never met her) drives me insane. 

I've been told by multiple other friends of his that all his friends including this girl all thought he was being stupid by not trying to fix things with me. She was apparently very happy to hear that we got back together. They still text occasionally and he'd like to visit her at some point. I've seen texts between them as I often use his phone for things. He doesn't try to hide anything about their friendship at all. I also know that he has no interest in her physically either- he's told me this and she is also really not his type physically at all (judging from myself and his exes). 

I literally do not have a single reason for hating her. She seems like a nice person from what I've heard. But every time he gets a text or he mentions her I feel angry, hurt and jealous. I think it could be because she's a friend that was around when I wasn't and was there for him at that time. But I hate feeling like this and I really don't want to be the girlfriend that dictates his friends. Does anyone have any advice on how to stop these feelings? I've thought about messaging her on Facebook or something just to talk but that just seems weird to me...