I need advice!

This might belong in r/relationships but I really want only the female perspective on this for now. The guy I'm seeing has an ex-girlfriend that really wants to get back with him and he's told me this. I felt good about the fact that he mentioned because I felt like he was being honest and just letting me know that she was trying to do this. Well yesterday I was gone alllll day at a concert and today I look at his fb and she's tagged a pic of him in his apartment, meaning she was over there. I freaked out because I don't trust this girl and I know her intentions are bad. She knows that he's seeing someone and I think she knows it's me but that won't stop her.

I texted him after I saw and said "So _____ came over yesterday? You didn't think maybe I should know that? WTF I get that you're not interested in her but clearly she's interested in you." To which he replied with "It's not that big of a deal. She's dating someone anyway. I didn't say anything bc I knew it would be blown up." I responded with "You said she's the type of person who doesn't care about that so it is a big deal" (he mentioned that she doesn't think being in a relationship has ever stopped other people from hooking up aka cheating...great girl) "You know how I feel about it so you hid it from me? If she has a boyfriend why is she hanging out with her ex? I don't want to argue about this because there's nothing to say to fix it but I'm fucking pissed. If you're gonna do something behind my back at least tell the people you're doing it with to hide it too." (Meaning don't put it on fb for me to discover). His response to that was "I wasn't hiding anything. You're acting like we fucked or something. Is it any different you still talk to ____?"

A side note here, we are not in a titled boyfriend girlfriend relationship, but we HAVE agreed that we are not seeing anyone else or sleeping with anyone else. I have an ex of a year and a half that I am still in some casual contact with but we do not hang out just the two of us EVER and I tell the new guy if we do talk about anything important. I think I'm just mostly hurt and confused because I don't want this guy to turn out like the rest of them. Am I reacting too crazy here or this justified? Thanks in advance for all your thoughts :)

tl;dr--The guy I'm talking to hung out with his ex girlfriend who wants him back and proceeded to hide it from me until I discovered it on facebook.