Donald, It Still Sounds Like....
...a vision issue, but who knows? Looking at the monitor for long periods of time can cause issues for many. I've found if I sit too close, or too far away, that eyestrain occurs for me too. I've set up my chairs so they are at a set distance and so that the mouse is flat and at a comfortable distance so my wrist isn't strained. It takes a little testing to get it just right. You didn't say, but did you have another computer at your home previously, or is this the first one? A day of work, then looking at the screen for long periods may be causing this?
Have you tried changing the screen resolution? Something larger or smaller based on the distance you are sitting from the screen. Incorrect text size can make reading difficult and cause headaches.
Hope this helps.