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The black spot is likely to be due to posterior vitreous detachment which is a normal ageing change. The vitreous gel in the back part of the eye separates from the retina and shrinks to form a clump which may appear as a black spot or floater. This change usually goes uneventful but sometimes it may cause a retinal tear. So it is important that you see an ophthalmologist to rule it out though it is not likely. Other possibilities for a black spot could be retinal hemorrhage, cataract etc.
Headache and tenderness around eye may be due to raised eye pressure, a condition called glaucoma. This condition also needs to be ruled out fast as raised eye pressure can harm the optic nerve of the eye.
Grittiness could be due to dryness and a lubricating drop like Refresh Tears should help you. Examination is important and you should see an ophthalmologist. Thanks and best wishes.
Follow-up questions, if any, are welcome.
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