I considered the caffeine withdrawal possibility also, but 2 weeks seems a long time for a caffeine withdrawal headache, especially since the clearance time for caffeine to be out of the body completely is much less than that. When I ditched the caffeine for a while, I had a mild headache for 2 or 3 days. Then again, I wasn't a heavy coffee drinker to begin with. Diet Coke, on the other hand....oy! A 2 liter bottle a day.
If I get a headache now, it's either because I'm not drinking enough water or it's stress/sinus acting up.