Pyelonephritis: This is an infection of the kidney, which can cause flank pain, chills, and nausea. The presence of these symptoms, especially if accompanied by fever and urinary symptoms like dysuria or frequency, makes pyelonephritis a leading consideration.

Nephrolithiasis (Kidney Stones): While kidney stones can cause severe pain, the presence of chills and nausea might suggest an associated infection or obstruction. Stones in the left kidney could cause left flank pain.
Diverticulitis: Inflammation of a diverticulum in the sigmoid colon, which is often located on the left side, can cause left lower quadrant pain, but can sometimes be felt in the flank. It may be accompanied by nausea and chills if there's an associated infection.
Gastroenteritis: An infection of the stomach and intestines can cause nausea, vomiting, and sometimes flank pain if the infection involves the kidneys or if there's severe dehydration.

Appendicitis (if atypical presentation): Although appendicitis typically presents with right lower quadrant pain, an atypically located appendix or a late presentation could potentially cause left flank pain. Missing this diagnosis could lead to perforation and severe complications.
Ectopic Pregnancy: In women of childbearing age, an ectopic pregnancy can cause flank pain, nausea, and chills due to internal bleeding. This is a medical emergency.
Aortic Dissection or Aneurysm: A tear or dilation of the aorta can cause severe flank pain and is a life-threatening condition. While less common, it's crucial not to miss this diagnosis.
Pneumonia (especially if involving the lower lobes): Lower lobe pneumonias can cause referred pain to the abdomen or flank area. Chills and nausea could accompany the infection.

Pancreatitis: Inflammation of the pancreas can cause severe abdominal pain that might radiate to the back or flank. While not typically the first consideration for flank pain, it's a possibility, especially if there's a history of gallstones or alcohol abuse.
Splenic Infarct or Rupture: Problems with the spleen, such as an infarct (death of splenic tissue due to lack of blood supply) or rupture, can cause left upper quadrant or flank pain. These conditions are less common but can be serious.
Inflammatory Bowel Disease (IBD) Flare: A flare of Crohn's disease or ulcerative colitis could potentially cause flank pain if the disease involves the upper parts of the colon or if there's an associated complication like a kidney stone.