Klara was hurt when Lucas told her that she shouldn't kiss him. In Klara's eyes, Lucas is an ideal man. Then the following lies she told are just side effect of the environment she's in and she doesn't realise how bad it was.
She had a crush on Lucs and even kissed him on the lips in one scene (something only to be done with parents as Lucas reminded her). Lucas had probably noticed this and would ignore or reject her behavior. So I guess she just grew mad or jealous and said those things.