Many things could explain that. You may be sensitive to the sun's UV rays, and since almost everyone goes out into the sunlight everyday, we're exposed to these UV rays (to a less degree since we have the Ozone layer, although it's partially depleted.)
Maybe it's people/society that's giving you these headaches? You need to be specific here - what specifically has been bothering you the most recently? If not, has anything been bothering you and been giving you these headaches for these past couple of years?
