My sister ped out today..?

We were walking in the cold, and all of a sudden she felt dizzy, and she ped out. Upon waking up she was crying, saying she felt nauseous. We brought her home and she said she felt cold and she ped out again. My parents brought her to the hospital, but I just wanted to know anyone's opinion of what it could be. She's currently on her period, she ate today, my mother's food, which never gets her sick. She's 17 years old. She's of normal weight, but she has big hips which makes her think she's fat. She might have irregular eating habits, but today she ate well, I saw her eat. I don't know if it's a bacteria, the oncoming of the swine flu, just give me any suggestions please? Thanks!