The most common source is a disc bulge or herniation pressing on a nerve root, but sciatica can also come from piriformis tension, spinal stenosis, or SI joint dysfunction — each of which requires a different approach. Guessing wrong here means treating the wrong problem for weeks.
A disc herniation on imaging can be alarming to see, but the finding itself often doesn't match the level of pain or disability someone experiences. Many people walk around with herniated discs and no pain at all, and many people with significant pain have discs that look relatively normal. What actually determines your recovery is less about the herniation itself and more about how your spine is loaded and controlled around it.