Hi there. since all your investigations are coming postive and no obvious etiology is detected, it is easier to understand that, the symptoms are showing are due to migraine headaches. Other causes could be stress headache, cluster headaches and a tension headache. Consult your neurologist for the needful. Take care.
Have you looked in to allergies, I know it sounds weird but you can also get headaches from an allergen, you might be getting a reaction from something your allergic to and not know it.