What causes it?
Pain around the eye is an incredibly common symptom. Why?
First, the cornea has the heaviest trigeminal innervation of any structure in the head.
Second, the cornea shares first division trigeminal innervation not only with its surrounding soft tissues, but with ethmoid and sphenoid sinuses, meninges above the tentorium cerebelli, and cranial base blood vessels. Perhaps because so much trigeminal traffic comes from the eye, trigeminal irritation as remote as the occipital meninges will be perceived as coming from the eye.
What to do?
Try to elicit a history that is characteristic of certain conditions, such as foreign body sensation for keratopathy and lancinating pain with trigger points for trigeminal neuralgia.
But history doesn't go a long way here. Your best hope is to find abnormal signs in the eye, its adnexae, and its cranial nerves.
When the clinical exam is unrevealing, cranial imagining may be necessary, the chief concern being a basal intracranial or sinus tumor.
In some cases, such as cluster headache and trigeminal neuralgia, there will be no abnormalities found on any diagnostic studies.
Acute or nagging periocular pain of uncertain cause should evaluated by an ophthalmologist. If ophthalmic examination is negative a neurologist should become involved. Imaging should be deferred until these clinical evaluations have been completed.