What should you do if you slept with a girl who belongs to someone else but you liked her and she liked you?
People can fall in and out of love, but, both of you were wrong in what you did. Sex alone doesn't make you fall in love so if you're betting a one night stand of sex is love forget it! If you loved each other and knew it then she should have broken it off with her boyfriend. When in doubt, both of you should put yourselves in her boyfriend's shoes. Not a nice feeling is it! Since you think you are in love then she is going to have to break-off with her boyfriend, but it would be wise not to hurt him further by telling him she has slept with you. If she isn't willing to do this then she doesn't love you enough.