Musculoskeletal Strain: This is the most likely diagnosis due to the specific trigger of lying down on a flat hard surface, which can exacerbate muscle strain or discomfort in the lower back, especially if the mattress does not provide adequate support for the spine.

Herniated Disc: Pressure on a herniated disc can cause pain, especially when lying down, as it can increase the pressure on the disc and surrounding nerves.
Degenerative Disc Disease: Similar to a herniated disc, degenerative disc disease can cause lower back pain that worsens with certain positions, including lying flat on a hard surface.
Sacroiliac Joint Dysfunction: The sacroiliac joints can become inflamed or irritated, leading to pain in the lower back that may be exacerbated by lying on a hard surface.

Cauda Equina Syndrome: Although rare, this condition involves nerve compression in the lower spine and can present with severe lower back pain, among other symptoms. It is a medical emergency requiring prompt intervention.
Spinal Infection (e.g., Osteomyelitis, Discitis): Infections of the spine can cause severe back pain and are critical to diagnose early to prevent serious complications.
Spinal Fracture: Especially in individuals with osteoporosis or those who have experienced trauma, a spinal fracture could cause pain that worsens with lying down.

Ankylosing Spondylitis: A form of arthritis that primarily affects the spine, although it can cause widespread pain, stiffness, and reduced mobility, its onset and progression might not directly correlate with the specific scenario of lying on a hard surface.
Spinal Tumors: Both benign and malignant tumors can cause back pain, but they are less common and might not have a direct relationship with positional pain unless they are compressing nerves or the spinal cord.