Ok, so I actually felt some sympathy for you until you got to the part of the story where you cheated on her several times. I think that if your girlfriend told you that she was drunk on her brother's couch then you should believe her. Perhaps your mistrust reflects your own actions and mistrust in yourself instead of in her. Because you have cheated, then you expect her to do the same. There needs to be trust in a relationship and it is not ok to cheat at all. If it is bothering you so much that it took her so long to call you back, then you should talk to her about it. Honestly is the best policy in a loving relationship and it is directly related to trust. Talk to her and let her know what is bothering you.