I hooked up with a girl at the club I was playing at even though I have a girlfriend.
That's not good. Can you tell me more about this?
First of all, don't you think that's rude, to do that to a partner of mine
You shouldn't sleep with other people if you have a girlfriend. It will damage the trust in your relationship, which can be hard to repair.
I'm not really a cheater and that was just a one time thing.
Even a one time thing counts as cheating. You are dating someone and sleeping with another person is a betrayal of the trust your girlfriend has on you. You need to be honest with your girlfriend about what you did.