Is it possible that not eating can cause you to feel sick when you do eat?!

My girlfriend didn't really eat much she would go days even up to a week with out eating (by choice n what I explain below) but since we got together I made her eat more n she is always sayin that she feels sick after she eats. Could this be because she didn't eve eat much n her body isn't used to it? Or what could it be? Please help n also i would like to use this to convince her to eat more so if u could help with that too it would help alot. Thanks, in advance.