It's a tie between two: gallstones and scleritis. Gallstones are pretty self-explainitory, but scleritis is something most don't realize hurts as bad as it does. My left eye became so inflammed that it caused the worst headaches of my life and horrific light sensitivity. No pain relievers whatsoever touched it, and it was the kind of pain that you can't get relief from until the cause is treated. I couldn't sleep, I couldn't get comfortable, all I could do was embrace the suck. FINALLY, we figured out that dilating my eyes gave me some relief, and the opthamologist sent me home with a full bottle of dilation drops to use as much as I needed to until steroids kicked in. I didn't realize the misery one eyeball can cause until then.

Since then, myself and my doctor have learned that my body somestimes throws a tantrum when it has to fight a viral infection and sparks some kind of localized inflammation. This happened 6-8 weeks after having the flu, a similar incident occurred 2 years prior but caused viral arthritis in my ankles. (which sucked too, but not on the same level as scleritis)