If you understood the ending to last night's season finale of Terminator, could you please enlighten me?

Ok...How is it that Cameron couldn't go through time with them? She did it just fine on her previous trips! I understand this is a different time, but why don't they know who John Connor is and are we supposed to believe that the Scottish Cyborg is posing as Cameron now? Because when he looked at her, he smiled, then is face turned to confusion....like mine.