Insanely jealous or every right to be?

I've been with my bf 3 years and when we first met I had met a bunch of his girl friends at a party he hosted introducing me as his gf that night. Fast forward two years into our relationship I notice he's always liking this one girl "friend" pictures and post on fb. I told him it bothered me when he was liking this one girl in particular selfies. Every single time. It was no big deal to him since they were just friends. Last year me and him got into a fight and I wanted to see him but he likes space when he's mad. Come to find out he was actually messaging her while he was avoiding me asking if she wanted to go for a smoke ride. My bf enjoys smoking with his friends which is fine but the fact that we're fighting in this one particular girl I don't like bothers me.
I found out by going in his fb and I don't think they actually met up and she messaged me too saying she's just a friend. But not even a week ago I went through his phone (spare your breath if your going to lecture me over that because I know I shouldn't but I did) and I saw he had messaged her on fb wishing her a happy birthday and saying he wishes he could see her but hope all is well. Should I be worried? Is this something I should bring up?
It just freaks me out because he only messages her when I'm not with him. I don't know if it's because he associates a fight between me and him if he communicates with her because I've asked him to not in the past because she's trouble. I know they don't talk often and my bf goes out of his way to be nice to everyone so I'm not sure if it's just me being insecure and leave it alone or if I should say something.
It's been bothering me and I don't want to cause a fight if it's just me being insecure I need someone's second opinion. I do believe they've never hooked up and only been friends a few years before I met him but I'm still bothered.

If you're at the point of looking at his phone, sounds like the relationship is over. And very unhealthy. It doesn't sound good that he's trying to hang out with this girl either when he knows you don't like it. Very disrespectful.

Didn't you already post about this previously, and we all told you to either trust your BF or not? You can't control who your boyfriend talks to or is friends with, so unless he has acted in a way that breaks your trust and oversteps boundaries by cheating on you, then you shouldn't worry about it. He has every right to have friends, male or female, just like you do. Have you met this friend of his? That may make you feel better about it if you get to know her, and maybe you'll become friends with her too.