University staff treats me badly

I'm a student of BSc Geography at the University of Northampton, and I liked my university until something happened.
All began when I stayed overnight with my girlfriend in her room in halls, because I didn't have a room at that time. She told me that it's an all-girls flat, but I thought that's a coincidence, and actually I was relieved because I'm not very outgoing and feel a bit uncomfortable with other guys around.
Then she went to London over the weekend, so I moved out. She asked me to apply for a room change for her because she didn't like her flatmates, they are all Chinese, but she said other Chinese girls often create groups and leave some outside, and so was she.
So I went to the student centre, and there unfortunately I mentioned that I stayed with her. They told me I'm not allowed to stay over night, especially not in her hall, because it's an all-girls hall. I didn't know all these things, so I didn't expect that I did something wrong. When I asked whether I'm allowed to visit her they said that's ok.
It happened that I couldn't find a room for that following night, so I wanted to stay in the IT centre over night which is allowed, but I had headaches so I wanted to ask a warden for an Aspirin. When I told her my name she became very rude and said she doesn't have any understanding for me, and that I'm not allowed near the halls. I went back to the IT centre, and there a girl asked me whether I'm ok, so I told her that I didn't have a room. She offered me that I could stay in her room because she had to work over night and wouldn't use her room. When we were on the way we met that warden who became very rude again and said to the girl I would be harassing her and that I'm not allowed near the halls. So she offered me I could stay in her sister's house.
A few weeks passed since that night, and I visited my ex-girlfriend again several times during daytime because I was told that's allowed. But this evening two wardens came to her room when I was there and told me I had to go, because no male students are allowed in that building at all. I was upset and told them I was told it's allowed to visit her during daytime. They said it's not. So I finally left the building.
When I was back in my own room I chatted with my ex-girlfriend online and she told me the warden I met several weeks before came to her room and told her I would be "playing with her emotions", and that she would be in trouble if she would let me in again. She also said other boys could visit her, just I couldn't.
So I'm confused and upset now, I have the feeling they just don't want me in the building. I feel bullied, and treated like a criminal, although I didn't do anything wrong morally. I might not have followed every rule, but only because I didn't know these rules, and my ex-girlfriend also didn't know.
She doesn't want to do anything about it, maybe because she's Chinese. She said I should leave it, but I feel everything is so unfair, I don't want to be treated like that. So what can I do? Is something like that normal at the University of Northampton? And what are the rules for visitors in halls?