I'm sorry, but in terms of anatomy, 'shadow behind the eye' doesn't mean anything. Probably your doc was trying to put something medical into common English, but only he/she knows what this refers to.
Whatever doc you have been referred to will have your medical records and perhaps some idea of why this phrase was used.
It's more the shadow I'm concerned about
i suggest you google this:
cluster headache
You might very well find something that helps you understand what you're experiencing.