Ladies & fellas...can you help explain this to me?

Ok I have a girlfriend i always tell her that she doesn't have to be afraid of my reaction if i should know something important. I told her many times before we got serious I'd rather hear something from her than someone else. Which leads to her telling me she was going to one of her friends birthday party I was perfectly fine with it...However come to find out it was her high school crush/ex boyfriend.They wear going to this lil private opera lounge party with like 13 people. Also, they took alot of pictures together but she messaged him on facebook telling him not to tag her. When he asked her why she said," it was complicated" and wouldn't say anything else. When I confronted her about this on why she lied. She told me," she was afraid of my reaction on going to her ex's birthday party. When, she obviously knows if she told me the truth from the beginning I wouldn't suspect anything between them. Furthermore, when i ask ehr,"why didn't you just tell him you didn't want to tag because you had a boyfriend and you was scared on how he was going to react?" she told me," because she doesn't want everyone to know her business". When i heared this i felt as if she thinks i'm gullable...maybe i'm wrong but for some reason i feel like there is something between them...So guys if this ever happen to you ," was you girl being truthful?"....Ladies: if you was in this situation idk like is it true? She told me nothing is going on between them....I told her if she wants to keep talking to him in private and hanging with him late at night she could do this happyly but I want no part of it....so i told basically to pick....and she picked me but for some reason i feel she could still be talking to him