Twice this. Got these headaches on day 13, thought it might be migraine but pickle juice was a pure miracle. If you don't have pickle juice or a tablet, slightly salted water might help as well. Never underestimate a hyponatremia (when you don't get enough sodium). Drinking water is important, but if you have changed your habits significantly it might destroy your water-salt balance (that's what happened to me - resulted in headaches).