Should i trust her ...?

I met this girl online and started talking , after about 6 months we finally met and became intimate .She is 7 years older than i and a divorcee.However we both liked each other and i asked her out . I thought that i knew everything about her(as she said she was an open book and had told me everything ) , however one day while we were sitting together , she was checking her mail and had to leave - i happened to snoop through her inbox and found messages with sexual content to a guy she had never mentioned before and were sent about 2 months back ...( this is the time when we had been talking over the net for about 4 months When i asked her about it she tried to find a way around but finally said that she was talking to this guy as well while we had been talking and theyd been having phone sex , however after meeting me she had not spoken to him and was only with me . Now i do not know whether to trust her or not ...please help ??