These symptoms sound remarkably like mine. I have the feelings of lethargy/fatigue, the drunk/hungover feeling and the feeling that my eyes aren't focussed, even though I can see everything fine. It's sort of like the feeling your eyes are trying to go in two different directions, like you get when you're mentally fatigued.
I've been suffering for it for the past 2 months and have had a CT scan and blood tests that have all come back negative.
Someone suggested Vertical Heterophoria Syndrome, which I am currently looking into.
I hope you find out what is wrong soon as I know how horrible this feels. I also know that it's difficult to get people to understand when there are no outwards signs of what you're feeling.