My 9 year old is constantly having headaches. when she is ill she tends to pass out. what can be the cause?

Hard to say: It could be as simple as a headache or increased brain pressure from a number of conditions/exposures. Proper evaluation will require a full review of history, meds, exam & maybe tests.Given that she passes out I would do this sooner rather than later.

Evaluation : There are many possible causes. I would advise seeing your pediatrician. Provide Motrin for pain relief after checking her temperature.