L2-L3 or L3-L4 Herniated Disc: This is the most likely diagnosis because the symptoms of bilateral flank pain and intense pain on hip flexion are consistent with a herniated disc at the L2-L3 or L3-L4 level, which can compress the lumbar plexus nerves, leading to pain in the lower back, flank, and hip areas.

L4-L5 Herniated Disc: A herniated disc at the L4-L5 level can also cause pain in the lower back, flank, and hip areas, especially if the disc is compressing the L4 or L5 nerve roots.
Lumbar Spondylosis: Degenerative changes in the lumbar spine, such as spondylosis, can cause pain and stiffness in the lower back, flank, and hip areas, especially with activities that involve hip flexion.
Piriformis Syndrome: Compression of the sciatic nerve by the piriformis muscle can cause pain in the buttock, hip, and flank areas, especially with activities that involve hip flexion.

Abdominal Aortic Aneurysm: A ruptured abdominal aortic aneurysm can cause severe, sudden-onset back and flank pain, and is a medical emergency that requires immediate attention.
Kidney Stones: Nephrolithiasis can cause severe, sudden-onset flank pain, and may radiate to the groin or hip area.
Spinal Epidural Abscess: A spinal epidural abscess is a rare but potentially life-threatening condition that can cause back and flank pain, fever, and neurological symptoms.

Lymphoma or Metastatic Cancer: Malignancies such as lymphoma or metastatic cancer can cause back and flank pain, especially if they involve the spine or retroperitoneal lymph nodes.
Retroperitoneal Fibrosis: A rare condition characterized by fibrosis and inflammation in the retroperitoneal space, which can cause back and flank pain, and may compress nearby structures such as the ureters or blood vessels.