In The Chronicles of Narnia, what made Lucy upset when she returned after her meeting with Mr. Tumnus from the land of Narnia?

Lucy is upset because no one will believe her story about meeting Mr. Tumnus. Lucy had taken tea with the faun, after which he briefly sent her to sleep by playing a little tune on his pan pipes. After Lucy suddenly wakes up, Mr. Tumnus confesses that he's a spy for the White Witch, and has been instructed to kidnap any human child that comes across his path and turn them over to her. Lucy pleads with him not to do this, and Mr. Tumnus agrees, even though he knows he will be in serious trouble if the White Witch finds out. Lucy and Mr. Tumnus part on good terms and Lucy quickly makes her way back to the wardrobe.
However, when she tells the others about her little adventure, no one believes a word she says. They think she's just making up a story. Lucy tries to convince Peter and Edmund of the reality of the magic land of Narnia, but they remain stubbornly unconvinced. Lucy is incredibly frustrated and upset by her brothers' treatment of her. She knows what she saw; she knows that Narnia is real.