She obviously doesn't trust you. That's her problem. If she really loved you she would try to make things work or at least try to understand what happened before making such a big decision. If u still want to be with her than u have to explain to her what really happened and reassure her that she's the ONLY person you love, and that u would never cheat on her. Hope this helps
It al depends where she caught you and how, if it was a bad setting she might of thought something bad, explain to her that she was your friend and have nothing to do with her, tell her that she is your only love and that you would never hurt her like that!