Had spinal fusion 14 weeks ago, severe burning pain when lying down at base of spine?

Nerve pain: Sounds like nerve irritation from the posterior rami. This probably is due to nerve injury prior to the fusion and the increased inflammation may have exacerbated the symptoms. I'd talk with your spine surgeon, you may need either Neurontin or Lyrica (pregabalin) to calm it down.

Post-op pain: You should see the surgeon who did the procedure and make sure you have xrays taken.