Also, it may be a sinus infection or ear infection. I'll tell you why, as relates to your eye dilating. One of my eyes has occasionally appeared dilated larger than the other for several years. I had looked it up, and they say variations like that are usually normal.
WELL, this morning I woke up, and I had slept very, very soundly on one side. I glanced in the mirror, and on that side, my eye was swollen and that eye was REAL dilated. In fact, my nose was kinda runny and my face in general was a little swollen. An hour or so later, when I looked in the mirror again, the eye was back to normal, matching the other one. So, I assume that the fluid, that had accumulated from solid sleep, had drained.
So, when I read your post, I immediately thought that maybe swelling on one side, which for me is sleeping on that side, and for you is causing headaches and eye pain, probably made it dilate that way! Swelling in the face means you have a collection of mucous or infection going on in one of the cavities of the face...kind of like if you cry, your eyes will swell.
I think before you go and have a CAT scan, you should consider visiting an ear nose throat doc, ENT, very easy sort of medical visit, and even tho your labs were okay, have him look in your ears and sinus cavities, to see if you've got swelling or whatever in them. Then he can track down the source.
If all the openings are clear, he would probably refer you to an opthalmologist who can look at the retina of that eye and see if anything more dramatic might be going on, that might indicate too much pressure on the eye, from a blood vessel situation or the brain, perhaps. Then you could go for a scan.
Try having a chiropractor look to see if your spine is in alignment. Mis-alignment of C1 or C2 vertebrae can cause some weird things to happen.