WHAT WAS MY GIRLFRIEND THINKING?!?

Last night when i came home my girlfriend was being "grumpy" on my facebook wall, not posting anything horrible, just grumpy comments. i then asked her what was wrong as there was obviously something wrong and she proceeded to tell me how she had 'found out i was chatting up one of her friends', (just to make it clear i was not chatting up her friends, she was just paranoid and bluffing to try and make me admit to it!!!) she told me that her friend had told her everything. because i was not chatting up her friends and was confused what her friend might have said to her i went on her facebook to see, i then saw SHE was chatting up one of the people who bullied me from my old high school! she was sending hearts and trying to steer the conversation towards ' and what not :/ i questioned her about this and she didn't really say much and tried to brush it off as if it was okay. i just don'tt know what to do, i do not feel like she trusts me, she thinks im chatting up random girls behind her back all the time and im not, im trying toorganizee a trip to france for me and her and im only 17 anddon'tt have a stable income around my education! i just feel like sheisn'tt appreciating what i do for her and idon'tt really know what to do :/ anyones view would be appreciated :/ thanks