Once sided headache at the temple area that appears suddenly over a day or two, or headache that develops gradually over several days or weeks, could be suggestive for temporal arteritis (an inflammation of blood vessels wall, inflammation of temporal artery).
Additional symptoms that develop gradually are scalp tenderness in temple area, pain in jaw joint while eating and talking, visual disturbances like feeling of shade covering the eye, double vision, that may lead to eye blindness.
It is possible to experience some nonspecific symptoms like tiredness, depression, night sweats, loss of appetite, loss of weight and fever, for weeks or even months before developing headache or visual disturbances.

See your physician!