There are a few reasons why your back might hurt every time you sit. It could be a sign of a muscle strain, poor posture, a herniated disc, or arthritis. If the pain is severe, it's best to see a doctor to rule out any serious conditions. Muscle strains and poor posture can usually be fixed with some simple lifestyle changes. However, if you have a herniated disc or arthritis, you may need to see a specialist for treatment.