Hi and thanks for the query.
This seems like a slipped disc in the lower lumbosacral spine that is causing impingement on the sciatic nerve or a fracture or dislocation of the coccyx, the last and the smallest bone in the spine. An MRI of the spine is warranted and a nerve conduction study (NCS) and electromyogram (EMG). This should be preceded by a Neurological examination to rule out nerve impingement. The MRI will clearly delineate between prolapsed discs, sciatic nerve impingement, soft tissue injury or coccyx dislocation/ fracture. It will also provide details on the sacroiliac joint which is easily dislocated but in your scenario this is unlikely.
I would suggest that for now, you also start Ibuprofen liquigel massages thrice a day and use a doughnut pillow to sit on while driving and prolonged periods of sitting. Try to avoid odd postures and sleep on an orthopedic mattress or on the floor to avoid odd posture while sleeping.
Let me know if there is any further information required.
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