What is the cause of back pain after sitting for a long time?

Hello doctor,

I am a 26-year-old male with a height of 69.69 inches and weighing 189.59 pounds. I used to be active playing soccer and volleyball but since my injury, I have been sedentary. I have pain when I sit for a long time, especially at work or driving, I tried many physiotherapy methods.

Below are the key points from the summary of my MRI:

1. Mild straightening with loss of normal lordosis of lumbar spine denoting lower back muscle spasm.

2. Diminished height with loss of normal bright T2 signal intensity of L5-S2 lumbar intervertebral disc in keeping with degenerative changes.

3. L5-S1 showed a diffuse posterior disc bulge with a focal central disc protrusion in addition to slight or minimal caudal migration of disc substance that indented epidural fat and thecal sac with minimal bilateral encroachment upon neural exits foramina and recesses. Moreover, linear bright T2 signal intensity seen within the protruded disc is suggestive of a tear.

4. Normal diameters of the bony lumbar spinal canal.

5. Normally portrayed vertebral bodies with preserved height and regular edges. Normal marrow signal of the vertebrae.

6. Normal appearance of the conus medullaris and cauda equina nerve roots.

7. No paraspinal soft tissue abnormalities

So basically I do have a disc that is protruding and some of the doctors and physiotherapists I have seen said that it is pinching a nerve. Others say that the protruded disc is not the cause of the pain, rather it is my own lack of flexibility and have suggested that I have sacroiliac disease. As for treatment, I have done physiotherapy to cure the protruded disc but of course, this takes time, in the meantime I continue to experience pain in my lower back and my legs. All doctors have ignored the slight tear suggesting it is not a problem, but the basis of the problem is the protruded disc. How do I identify the source of the pain?

Kindly help.