In episode 12, 4:58 there is a part were Lelouch and C.C are fighting, then the scene changes, C.C seems to be lying unconscious, while Lelouch is mad and taking a shower.
What happened there? Did he kill her? Beat her? Rape her?
She isn't sleeping or unconscious as after Lelouch calls Ohgi we see C.C is awake (6:15), remember, this is C.C, she will usually lounge around the place eating Pizza.
Lelouch learned moments before that his actions at the Battle of Narita caused the death of Shirley's father
Conversation between Lelouch and Shirley:
Shirley: Lou Lou, tell me. Zero, he fights for the weak doesn't he?
Lelouch: What? Yea, well that's what he says
Shirley: Then why did he kill my father? You know my father, was so gentle. He never ever hurt me. He didn't do anything wrong. But, he was buried alive. He couldn't breathe. Why? Why did my father have to die?! I don't want this! Please, Lou Lou. Help me.
this shakes Lelouch's resolve after he realizes his actions caused someone's death but the effects of that death didn't stop at that person dying, C.C reminds him of what he said to Taizo Kirihara in the Mt. Fuji Mines and also reminds him that he has already killed many times before, rebuking the way he is acting now after he truly understands what it means to be responsible for taking lives.
Lelouch pins down C.C in anger because he didn't want to hear the truth, claiming he was prepared for this the moment he killed his half brother Clovis, yet C.C is saying he wasn't and he need to be now cause Shirley wont be the only person close to Lelouch who will be effected by Zero's war against Britannia.
Just as the scene cuts we hear a steel door, while there are no steel doors in the estate this is more of a metaphor of Lelouch locking himself away as he reflects what he has done, this is why he is in the shower banging his fist into the wall as the truth C.C said is something he still couldn't accept.
As for what happened just before the cut, well the steel door is also probably indicating Lelouch stormed out, but then why is C.C just laying there not moving, well also remember she is an Immortal so she's probably kept herself at arms length to people and tried not to understand them, she probably didn't realize that while she was trying to help Lelouch's shaken resolve she was hurting him as well and is probably thinking on weather she was too harsh