The causes of pain are - allergies, inflammation of eyelids, contact lens problem, corneal abrasion, blocked tear duct, dry eyes, foreign object in eye, pink eye, stye, chalazion, ectropion, entropion, cluster headache, injury, iritis, etc.
So, will have to rule out any of these possibilities and as per that can give you treatment. For that share more details pertaining to your problem.
Get your eyesight tested.
Medication:
Meantime, use Euphrasia eye drops twice daily.