Hi there, yet more advice....
Have your eyes checked. Even if you wear corrective lenses or contacts. your prescription may need to be updated.
That you get about the same time of day leads me to suspect it is your eyes. I suspect also that after lunch you regularly engage in something or maybe you do at lunch, like reading the COMPUTER for too long of time without taking microbreaks.
Every 15 mnutes look across the room and focus your eyes on a spot or object for about 20 seconds. This will help out a lot.
Another good suggestion that was posted is to cut down on caffeine, but be careful if you are a regular coffee/tea drinker cause if you cut down too much too fast you may get addiction headaches too.
-Eduardo
PS: Stop asking a bunch of non-medical muggles on the Net and go see a real doctor for goodness sake it could be the sign of somethng very serious like hypertension (high-blood pressure) or god-forbid a cancerous growth.