Help! Need to stop not eating in 3 minutes!?

OKay so my mom wants to take me out to lunch but lately I have been barely eating, yesterday all i had was a chewy bar until like 5 pm and then i only had half a bowl of pasta. i tell my parents i'm eating, but i feel natious when i eat too much. my mom wants to take me out to get a giant burger and fries, and i agreed w/out thinking. ideas on how to eat lunch without getting nautious! DON'T tell me to go to a doctor, okay?