You need to check if there is a change in your refraction. Get your eyes checked by good ophthalmologist for refraction and dry eyes. Computer vision syndrome is a common condition nowadays due to overuse of computers. If your refraction is correct and if you don't havedry eyes then you have to change certain work habits while using your computer like , adjusting your posture, your viewing distance, your viewing angle, air flow in your office etc to avoid these issues. List is long, just go in net and find out.