Your chair isn't offering proper lumbar support.
If your chair has a lumbar adjustment, make sure it's properly adjusted to fit your body. If it's not adjustable, consider purchasing a lumbar support (or make your own with a rolled towel or pillow). See Chair Setup and Usage.
You're not maintaining good posture.
Proper posture is essential, and helps minimize the strain on your muscles and joints. See Chair Setup and Usage. Your chair is too high or too low. Dangling feet that don't reach the floor or a footrest can put strain on your lower back. Make sure your chair is properly adjusted and your feet are supported. See Chair Setup and Usage.
You've been sitting too long without a break.
Even getting up for a drink of water can help provide the change of work your body needs to feel comfortable. You may also want to consider stretches to help relax your muscles and improve circulation. See Stretches and Exercises.
Your chair isn't adjusted properly.
If your chair has an adjustable backrest, tilt, seat pan, or lumbar support, make sure they're adjusted to correctly fit you. See Chair Setup and Usage.
You're leaning forward to view the screen.
People who wear glasses often sit in awkward postures or lean forward to see properly, resulting in neck and back pain. Even those with naturally perfect vision sometimes adopt this position out of habit. Always ensure that you're using proper posture.