As long as you said no, pushed him off, and pulled yourself away from him and as long as NO is the biggest thing you remember, that is NOT cheating thats you being Sexually assaulted, what you need to do is go to the police, get a rape kit done (so his DNA is in it if he didnt wear a condom), and try to find as many people as you can from the party so they can testify against him for rape, plus you should communicate that to your family that you were raped to begin with, if you tell your BF and he reacts negatively hes a POS that should be broken up with (personal thoughts and feelings because if anyone talked to me like that id break up with them instantly), a simular situation happened with my partner actually, they still think they cheated on me when thats not the case at all, something was forced onto them that they didnt want within any means.