My girlfriend is believing rumours about me hooking up with another girl? I need advice?

My girlfriend has heard rumours from some low lifes, that I have been cheating on her. I went to a club saturday night with my mates and someone sent my girlfriend a message saying that I went home with another girl last night. Thing is I did sleep with someone else when we were together a yr ago (it's a long story), but she forgave me and ever since then I've been trying to make it up to her, and we've been doing great! And then this happens. I don't know who it could be, but they are trying to break us apart, because it isn't the first time either. The person always sends these texts from an unknown mobile phone. So we can't trace it, but it's causing stress on our relationship. The thing is this person who is sending the messages, said that I was all over some girl and that we got into a taxi together. Yes I did get into a taxi with a girl, but she was a friend from college who I saw in the club and me and my mate shared the taxi with her and her mates, but that's all. So I don't understand why this person is saying that they saw me kissing, and fondling this girl. None of that is true and it really pisses me off! Because everything was going ok, until this and now she won't talk to me, and she even believes word of mouth from some stranger instead of me. I know I cheated in the past, but I have been faithful ever since, but now I feel this is the end. I actually cried to myself earlier and I know that's not manly lol, but that's how strongly I feel about her and people keep trying there damned hardest to split us up! I don't understand it. Also I'm 21 and she is 20