You say you were busy working, did that involve a lot of looking down ? Also when people are working hard and over long hours, they fail to eat properly – would that have happened in your case ? Stress sure messes up the body chemistry.
Did you suffer stomach problems by any chance ? Did you regularly take antacids or painkillers ? I am curious because these, over time, deplete magnesium, which can in itself cause headaches (and migraines, incidentally). Check out the info on Magnesium:-
Your description of the head pain/discomfort is familiar and quite typical in our community, very suggestive of CSF (CerebroSpinal Fluid) build-up/flow problems. Your eyes can tell you a lot about the condition of your blood vessels and can also indicate whether the pressure in your head is too high, see post at:-
Venous drainage (from the head) takes a different path when you are laid down, so this, along with gravity being on your side, may be the reason why laying down sometimes relieves your headache. Which kind of headache is relieved by laying down, have you noticed ?