Sometimes, when im about to sleep in prone or supine position.. it feels like theres a hot pack stick at my back. its hot and uncomfortable.why?

SIJ: 22 yo male bemoans "hot poker stick" type of pain in low back region when reclined supine or prone. In these positions the anterior and posterior superior iliac spines contact the mattress/floor thereby torquing the pelvis into horizontal alignment; thereby challenging the lower lumbar vertebra with mechanical stress; imparting strain to the L-5/S-1 joint; raising concern for spondylolisthesis.