What is the cause of Tracy's frequent nosebleeds in the novel, Tracy Flick Can’t Win?

Tracy's nosebleeds are caused by anxiety. Shortly after her worrying interview with Buzz Bramwell, Tracy's nose starts to bleed regularly. She insists that the interview is not troubling her, yet the nosebleeds suggest otherwise. On the night she confesses her secret to Marissa, her nose also starts to gush blood, thus conveying her internal distress.