headache related to sun could be of many reasons and to my knowledge I have seen sinus headaches are triggered by exposure to hot sun and even bending forward is another trigger in this condition- a sense of heaviness is felt. Did you consult ENT regarding- a sinus xray may be needed. Other causes include sometimes tension or muscle contraction headache and even migraines are precipitated by sun light- they d have accompanying nausea and sometimes vomiting. Let me tell you something- try avoiding exposure and if needed you can use a hat or umbrella.