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The lower back pain can be due to two most common reason.
First one is the condition associated with the vertebral column and spinal cord that results form disc degeneration and compresses nerve fibers of the spinal cord.
Next one is the condition associated with renal system either urinary tract infection
or renal stones
As you are having swelling and chill on the left back sided which is less likely to be due to vertebral problem and more likely due to renal infection
or stones and you might be having hydronephrosis
. Additionally there can be chances of gluteal abscess
which is the abscess formation under the gluteal region,
It is recommended to visit a doctor for detailed check up and radiological test as you may require antibiotic treatment. You can use NSAIDs for pain relief.