Do you usually ride when it's really sunny? (I mean, duh, who chooses to usually ride in the rain...)

Flashing light is a migraine trigger for me and I got sick driving home from school every day for months before we figured out that it was the angle of the sun through the trees in the afternoon.

Or I would second the tension idea. What is your frame made of? Do you feel pain anywhere else (wrists, back, ...)?