why did she freak about the sex at the end?

I think she looks likes Karen Fineman. That is the woman on Seinfeld , who alternates between looking pretty and ugly. I think it was The Festivus episode.

She was having a conflict of conscience. She had already decided earlier after the farm episode that she had to stop seeing Eric as she was afraid of what would happen if they were discovered. But back in the real world as teacher and student, she realizes she can't stop herself from thinking about him and wondering if he's with other girls. Later when invites Eric to her place to see her for the lat time, she started battling with her own morality. She becomes hysterical and starts crying for him because she thought she'd lost him and that he'd lost interest in her. But then her common sense kicked in again briefly and she kind of pushes him away as he tries to comfort her. But as soon as he wants to leave she freaks out again and is willing to do anything to keep him there. She gets confused and can't seem to decide if her fear of losing her job and possibly ending up in jail is worth losing him. He's also confused and decides he's had enough and leaves.