Renal colic is severe pain in lower back or sides. The pain is usually on one side, but may be on both sides of the lower back. Renal colic may start quickly, come and go, and become worse over time. It is caused by a blockage in the urinary tract. The most common cause of blockage in the urinary tract is accumulation of crystals like calcium oxalate, calcium phosphate and urates mainly, which are formed due to super-saturation of urine. Blood clots, ureter spasms, and dead tissue may also block your urinary tract.