I keep getting light headed and other symptoms including passing out and heasaches. Doctors can't find anything wrong. What could it be?

Syncope. I cannot tell from this question what testing has been performed so far. Full cardiac evaluation may include a 24 hour holter monitor looking for abnormal heart rhythms, even a 30 day monitor if the 24 hour monitor does not catch any episodes, sometimes even a tilt table test by a cardiologist, or even an implantable loop recorder are other diagnostic tools available with a cardiologist.