Since I was hospital I have cut alcohol and caffeine. The alcohol was not a problem, as I rarely drank. However, I have been a regular 2-4 cup per day consumer of coffee. These past few days I have been drinking a lot of water, juice, and a little herbal tea.
The official explanation of what is going on is that when caffeine intake is reduced, the body becomes oversensitive to adenosine. In response to this oversensitiveness, blood pressure drops dramatically, causing an excess of blood in the head (though not necessarily on the brain), leading to a headache. My headaches have been pretty mild, but I miss that coffee aroma and taste.