it's probably a mixture of everything your doing.
the 2 litres of water is a little extreme, make sure that you don't drink more then 250ml in one go, space it out through the day or you could dilute your blood too quickly and it can cause headaches (in extreme cases death, but that would be if you drank the whole 2 Litres in one go)
if your very active, then the lack of food/energy could also be causing you headaches.
and it could also be withdrawal symptoms from the lack of caffeine, if you were a heavy drinker then it's probably withdrawal symptoms.
if it persists for longer then a few days, or the headaches last more then a few hours, you should see a doctor.