Eyestrain caused by staring at a monitor for many hours is common, and easily explained. Human eyes are designed/expected to ordinarily focus on objects ten to twenty feet away (hence "20/20 vision").
In contrast, the monitor connected to your computer is typically positioned two to four feet away from your eyes. Your eyes are perfectly capable of focusing this close, of course -- it's not even difficult. But after several hours of this, the muscles that control eye movement get tired and sore. You feel like you're fine because your center of focus is flicking from one part of your display to another, or because you're looking at stuff on your desk from time to time, but those are still too close to give your eyes a proper rest. Neither glasses nor contact lenses contribute to or subtract from this problem.
Compounding the problem is the fact that most monitors are of the CRT type, emitting light internally. (LED monitors do this too, but at far lower light levels.) This light, especially the ultraviolet light, is tolerably dim for a few hours but painful for the retina and iris muscles after a while. Dimming the monitor is an option, but isn't necessarily practical for everyone.
Fortunately, the typical work day is divided into two four-hour shifts with a lunch hour in between. I try to take that lunch break away from my desk; even reading a book is easier on the eyes than staring at a bright monitor for that hour.