I totally understand your concerns about your daughter s health. the following points should be taken into consideration: 1- being thirsty is an important clue that the body is dehydrated. the big picture of fatigue, dehydration and feeling sleepy is usually associated with an infection, most likely a viral infection. regarding vagal syncope, it is more likely to happen in the case of fatigue and hence she is more likely to experience this problem more now, along with another possibility which is orthostatic hypotension due to dehydration that might lead also to syncope or feeling as if she s gonna faint due to decreased blood pressure when standing. 2- the following things are essential in the management of your daughter: A- keep her hydrated, monitor her urine output and the color of her urine : if the urine is dark and the amount has decreased along with being thirsty warrants hydration B- monitor her vital signs: temperature is essential to detect fever even if low grade, blood pressure and pulse also are important to see her well being. C- look for any possible site of infection: cough, runny nose, sore throat... if she does not get better in 3 to 7 days then she warrants more evaluation with blood tests. best regards