Probably from sitting at the computer all day.
Do you sit too close to the monitor? Lean forward closer to it? If so, try leaning back away from the monitor. Move your chair back a little. If your chair tilts back the way mine does, tilt away from the monitor.
Take breaks, get up and move around. Close your eyes for a few minutes.
Get an anti-glare screen. It helps a lot with less eye irritation. I don't know if I could live without mine.