Big problem?

me and my girlfriend are 16, and in a ldr. I had a feeling she might be cheating on me so I decided to go to her myspace page and look at the guys she had on there one was on there so I asked if he new Amanda. He said yeah and why? I told him she was my girlfriend and he said that is not what she told him. She had been cheating on me with him. She always told me she loved me and she would move and marry me when we turned 18 she said the same thing to him. Everything she told me she told him. SO i called her last nite to ask if she was cheating and she said NO. I asked her if she knew a guy named Dietrich and she said she dated one named Dietrich 4 years ago. So me, the guy, and her had a three way call and she didn't know he was on there, all of a sudden he chimed in and she hung up the phone immediately. Me and him were talking and she beeped him in and came up with a bullshit story about that was her sister that answered the phone and that she doesn't know me or anyone named zack.