Was I cheated on? I would appreciate input from people who have been in similar situations..

So I'd been dating this girl for about 9 months. We lived two hours apart and would only see each other during weekends. One day we were having an argument on the telephone about how I didn't do enough to come see her more often following which she told me she had something to tell me that might upset me. She proceeded to tell me that three months earlier she was "assaulted" by someone she thought was her friend. My heart started pounding as I saw the image in my head of some guy sexually assaulting my girl. While trying to contain my anger I asked her who did it. She told me it was Eric. According to her Eric was an old friend whom she met at a yoga class. She and Eric went out a few times to catch up on old times. One night they went out drinking to some bar and got pretty wasted, so instead of walking home so late at night she decided to crash at Eric's place as it was just across the street. After they got to his apartment they were sitting on the bed listening to music and next thing she remembered, she told me, was him undressing her. She said she froze and could not do anything. She was stunned because she considered him a friend and did not expect that, she tells me. She told me about it three months later. I spoke to Eric (though she didn't want me to) and he confirmed the story saying they were really drunk and when they got to his place they started kissing and then had sex. He said he was surprised to hear her negative reaction in the following day. She said she was going through some stressful times and was not ready to talk about it to anyone. It's been two years since then and we almost broke up over it and there have been some good and some bad times after that, but that particular thing has always bugged me. The story does not make sense to me. She tells me I would not understand because I can't put myself in her shoes.
My question is: was I cheated on? I would appreciate input from people who have been in similar situations on either side.