Symphysis Pubis Dysfunction (SPD): This condition is characterized by inflammation and instability of the symphysis pubis joint, which can cause pain when walking, especially with certain movements like walking backwards. The pain is often more pronounced with activities that involve twisting or bending, and walking backwards can exacerbate this due to the altered gait and stress on the pelvic joints.

Postpartum Pelvic Girdle Pain: Similar to SPD, this condition involves pain in the pelvic area but can be more generalized. The pain when walking backwards could be due to the specific strain this movement puts on the pelvic girdle.
Peroneal Tendonitis: Although less directly related to postpartum conditions, peroneal tendonitis could cause pain on the outside of the ankle or foot, which might be more noticeable when walking backwards due to the altered biomechanics of the foot and ankle during this movement.

Deep Vein Thrombosis (DVT): While DVT typically presents with swelling, redness, and pain in one leg, in rare cases, it could cause pain that worsens with certain movements. It's crucial to consider DVT due to its potential for severe complications, such as pulmonary embolism.
Osteonecrosis of the Femoral Head: This condition, although rare, can be a complication of pregnancy due to hormonal changes and increased pressure on the hip joint. It could cause hip pain that might be exacerbated by certain movements, including walking backwards.

Stress Fractures: Stress fractures in the pelvis or lower extremities could cause pain that is exacerbated by walking, potentially more so when walking backwards due to the altered stress patterns on the bones.
Piriformis Syndrome: This condition involves the piriformis muscle compressing the sciatic nerve, leading to pain in the buttocks and legs. While it's less common in the postpartum period, it could be considered if other diagnoses are ruled out, especially if the pain radiates down the leg.