Why I Am Getting Headache If I Did Not Have Food For 2 Hours

My almost 16 year old cant go longer then 2 hours with out eating or drinking without geting a headache, sick stomach and feel unwell? She drinks 5 16 oz glasses of water just during her school day and much more at home. She is healthy, very fit and active, however in unactive periods she does this. She had cyclic vomiting syndrome as a small child/kid but has had no episodes of this since she was in 8th grade. She will be in 11th grade after summer.