Urinary Tract Infection (UTI) or Pyelonephritis: The presence of chills, lower back pain, and blood in the urine after a stent placement is highly suggestive of a UTI or pyelonephritis. The stent can introduce bacteria into the urinary system, leading to infection. The symptoms presented are classic for an upper urinary tract infection.

Stent Occlusion or Blockage: A blockage of the stent can cause obstructive uropathy, leading to pain, infection, and potentially hematuria (blood in urine). This is a common complication of stent placement.
Nephrolithiasis (Kidney Stones): The stent placement could have been done for kidney stones, and if a stone is still present or if the stent is causing irritation, it could lead to the symptoms described.
Stent Migration or Malposition: If the stent has moved from its original position or was not placed correctly, it could cause irritation, obstruction, or infection, leading to the symptoms.

Sepsis: Although less likely, if the infection is severe and has entered the bloodstream, it could lead to sepsis, a life-threatening condition. The presence of chills and potentially fever should prompt consideration of sepsis.
Perinephric Abscess: An infection that has spread from the kidney to the surrounding tissue can form an abscess, which is a serious condition requiring prompt drainage.
Urosepsis: A severe infection of the urinary tract that has entered the bloodstream, potentially leading to septic shock.

Fungal Infection: Although rare, especially in immunocompetent patients, fungal infections can occur, especially if the patient has been on long-term antibiotics or has other risk factors.
Tuberculosis (TB) of the Urinary Tract: A rare cause of urinary symptoms, including hematuria and pain, especially in endemic areas or in patients with risk factors for TB.
Malignancy: Very rarely, the symptoms could be related to a malignancy, such as a tumor causing obstruction or bleeding. However, this would be an uncommon initial presentation without other symptoms or risk factors.