What could be the cause of pain in left side of the waist?

What could be the cause of pain in left side of the waist? Search from: Freebasics
Flank pain refers to discomfort in your upper abdomen or back and sides. … However, constant or severe flank pain may indicate a serious medical condition, such as dehydration or a urinary tract infection. Kidney stones or another kidney problem may also cause persistent flank pain.