most likely you answered a question about her dependency status incorrectly.
the one i'm thinking about is her residency status, she's regarded as living with you if her principal residence is the same as yours for more than 1/2 the year. absences for attending school, being on vacation, and the like are ignored.
the only other one would be if she was married and filed a joint return, but you give no indication she was married and filed a joint return with her spouse.
