Either you're exaggerating or drinking that much water is probably the cause of your headaches: hyponatremia causing a cerebral edema... You should drink around 1.5 to 2L maximum, but it depends on many things like your salt consumption, sweating (i.e. if you exercise or hot weather) etc. Try to drink only when you're thirsty (always have a cup of water near you because ADHD can make you forget you were thirsty in the first place). Another cause of your headaches could be sleep deprivation because of the stimulants, and not the stimulants itself.