The James-Lange theory states that emotions are the result of physical reactions to a stimulus. In Leilani's case, her physical reactions to the letter - her heart beating faster, her smiling widely and her dancing - would create the emotion of excitement.
The Cannon-Bard theory suggests that emotions and physical reactions to a stimulus occur simultaneously. Leilani's excitement would be accompanied by her physical reactions of a racing mind, a fast heart rate, a wide smile, and dancing.
The Schachter-Singer Two Factor Theory suggests that emotions are the result of a combination of a physiological arousal and a cognitive label. In Leilani's case, her physical reaction to the letter (heart beating faster, etc.) would create an arousal, and her cognitive label would be excitement as she thinks about all the things she could have done to get the award.
The Facial Feedback Theory states that emotions can be influenced by facial expressions. In Leilani's case, her wide smile and dancing would create a feeling of excitement, thus reinforcing her emotional reaction to the letter.