Working in a chair for 12 hours is causing me back pain?what could it be? slip-disc?

Possible: Sitting position puts a lot of stress on the discs but there are other painful structures on the back such as muscles, joints and ligaments. www.otwithmd.com.

Too much sitting: Sitting, especially unsupported, causes increased stress on the lumbar discs. This is a common cause of back pain. Getting up periodically can help as can rolling up a towel and throwing it between the chair and your lower back to give better support. Trying different ergonomic chairs can also be helpful but expensive. If all else fails, try some pilates/piyo/yoga to strengthen your core.