Sitting for long periods of time puts pressure on your back. More specifically your lower back discs. Sitting for long periods of time can cause pain in your lower back or aggravate more if you already have back pain. Sitting for a while also leads to slouching and bad posture which also causes pain. Be mindful to keep your back straight when sitting. If your job requires you to sit for long periods of time, try to get up and walk around for a minute or two every 2 hours or so to get your blood flow moving and release the tension in your back muscles. You could also invest in cushions for your chair to support your back.