Lower back pain worse when lying down. Worse when sitting. Located almost in buttocks. Does not radiate. What could it be? Is ice or heat better?

Pelvis joint. Female with low back pain & sciatica worse sitting/lying likely has unilateral sacroiliac joint subluxation; whereby lumbar nerves are tugged & impinged where they exit lumbar vertebra foramina. Sacroiliac joint instability is highly associated with sciatica. See comments: .