Easter egg: Go to the "special features" menu, then hover over the area just to the right of "Turn disc over". You'll see a circled "13" appear. Clicking on this takes you to the improvised "box of drugs" deleted scene.
Easter egg: The word Thirteen appears very briefly (a couple of seconds at most) as the main menu loads but is not clickable. However a video of the "Zen Chicken" can be accessed by disassembling the DVD to your PC. You can then save the video as a separate file.
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.