she was clearly locked into her story which was given very calmly the day after and so her only ability to "help" her finance was to appear so emotionally distraught at trial that the defense attorney could try to impeach her testimony because of her severe emotional state which rendered her unable to remember clearly. But it makes no logical sense to be MORE upset much later than right when it happened, especially since essentially nothing happened to her anyway. The only explanation for her state at trial, if it wasn't an act, would be her having to testify against her fiance but that would not affect her memory and so does nothing to impeach her testimony.
She was in very good spirits when she was interviewed by police. I find this quite odd and suspect the demeanor in court was good acting skills.