OK I need honest, and upfront answers regarding this:
My girlfriend has continued to have contact with her ex behind my back. Mails, FB chats, phone calls, small coffees, etc. Without me knowing about it or she ever thinking of maybe telling me...
Somehow in an argument this guy´s name came up. I ask her: "who the hell is he..." She tells me several times: "it is a friend, nothing to worry about..." I feel something is wrong and I keep pushing, and after a while she tells me, "well that guy is my ex, but he is just a friend nothing to worry about..."
I asked her if at any point the last months she ever felt the need to tell me she is still seeing him... She tells me: "you never asked about him... and I did nothing that would upset or hurt you..."
But at the same time she did asks me several times about all the women around me... But hey, she just asked me if they ever meant anything to me... But she never asked me if I had sex with them... 
Please elaborate... WTF should I do? Who is right?
My thoughts...
1) your a pussy . for one, you need to send this link to her so she can see what some online folks have to say about this situation? its like you cant talk for yourself.
2) shes a slut. of all the men in the world, if she is having small coffees (translate:blowjob in the car) with her EX, and didnt feel the need to tell you, then thats not a girl i'd like to be with
3)You both sound immature.. her questioning you constantly about women around you, and you saying shit like "But hey, she just asked me if they ever meant anything to me... But she never asked me if I had sex with them... " like a 5 yrd old boy trying to sound cool like you cheat behind your GF's back, but trying to sound mysterious
4)your insinuating you cheated on your gf, and your gf is meeting her ex, this is a dysfunctional relationship
5) there is no trust between you guys. trust is one thing that once its breached, it will always leave a nagging voice in the bak of your head saying " is she really with her gf, or is she meeting the ex again?"
WTF should you guys do? go your seperate ways, find new mates