What is this sharp stabbing pain in my left lower back, right above the waist.?

When it happens, it feels like i just got stabbed by a small knife, its enough to nearly drop me to my knees. What makes it odd after reading a ton of stuff about kidney stones is that the pain lasts for about 1/2 a second. Its literally gone right as i feel it. I've felt it about 10 different times in the last couple weeks. Never more than once a day, often its once every couple days. Its only happened while i was walking, and once while i was doing a seated lat row exercise.