Why did a 5 year old stop eating?

Iam a nanny for a 5 year old little girl and a 3 year old boy. 4 weeks ago i noticed that the 5 year old started to not want to eat lunch anymore. and then about 2 weeks ago she stoped eating all together. she said her throat hurt and her belly. so i took her to the doctor and he said she was fine. now she will eat alittle bit but not like she use too. she was one of those kids that would want 2nds and now im lucky to get her to eat a half of a pb and j at lunch time...any ideas?