Audio problem: During the girls' follow-up discussion after Tracy has sex with Ravi, Evie laughs and says, "What? We didn't go over that one." However, her lips aren't moving when she speaks the second sentence; the second part was obviously dubbed in later.
Continuity mistake: When Tracy first goes to meet Evie, she gets on the bus with no purse. She even leaves her house without one. When she sees Evie in the store, she has a purse on her shoulder.
Answer: I think what started it was her tongue piercing. She didn't want to eat cause it would hurt and her mom would notice.
I think it's a combination of the tongue piercing and Tracy's new rebellious attitude. She wants to be "difficult" by refusing to eat what her mom cooks. Evie, meanwhile, is trying to impress Tracy's family, because she wants to live there for as long as possible. So she eats whatever food Tracy's mom cooks.
Drugs will affect your appetite.