I think there might be something in the electrolytes or potassium or sodium thing. I used to get headaches sometimes right in the center of my forehead, sometimes all over. I started drinking more water in general, so maybe in addition to drinking immediately before, during and after, drink more water in the days preceding your ride(s). For me now that's basically all week.

I've also been experimenting with electrolyte replacement during and immediately after a workout. Not only has it staved off that horrible feeling of running out of gas, but it's also helped prevent me from being completely useless for the rest of the day. I like gels so far for during a workout, with a recovery shake (powder in water) after. You mentioned drinking only water while riding; does Gatorade/Powerade not sit well with you? If not, consider bringing a little something with you, any food you feel you can ingest. That's what I learned, anyway. Then again, it could just be your eyes and none of the things I mentioned.