One of my ex girlfriends did the same thing.... Lets see....
Oh yeah, she was working at the Library here in town and a contracted Networking company was out running cable... I happened to be there and my gf was telling me how this guy was asking some of the other girls there about her.. The other girls thought this guy was cute and stuff like that.... Lets see he was 27 and she was 21... Anyways, he had apparently bought her flowers ac couple times and brought them to work. Then one day (because the ex was super religious) was asking her about her church and then asked to have coffee with him to discuss the church in more detail because he wanted to be a part of it (Yeah right!)....
So anyways, I always drove her to our college class we had together, and on the way to class she said that she wouldn?t need a ride home because she was meeting a friend for coffee.... I asked who and she said Christina (The guys name was Chris by the way) and I was like Oh.... She was a horrible liar, or I'm just good at telling when people are lying. So after class got out I offered to stick around incase her friend didn?t show up so that she had a ride (pretty slick of me huh) and she got all paranoid and finally told me she was having coffee with Chris and that they were only going to be discussing her church and stuff....
Lets fast-forward a couple weeks to Valentines Day where the gf and I had plans and reservations and I had gifts and roses.... I tried calling her Cell phone and Home phone for a good part of the night and never got an answer.... I was stupid at the time and decided to go by her house and there she is with this guy making out in his car in her driveway.....
The next 2 girls I dated after that cheated on me with older guys as well... But those were like little one-month relationships if you can even call them relationships....
So it took me a while to trust my current girlfriend but we have been together for 2 and a half years and things are great. She has some guy friends, I trust her completely with them especially since I am invited to hang out with them and their girlfriends a lot of times.
Anyways, I don't want to make you paranoid or anything, but I think should be able to pick up on things if she is lying to you about things.... If you know her well enough then the signs should be there.