Still sounds like something with the kidneys. Are you on any medications? Some medications can cause kidney pain, even after you have been on them for a while and have been fine. Some acid reflux meds can do that. Best to be checked out by a Dr. Also examine your lifestyle. You may be abusing yourself with something, food, drugs, alcohol, etc. Everyone has a different tolerance.
I also have a history of kidney stones, still have one stone sitting in my kidney. I also have several back problems. I have recently had pains left flank as you described, also sometimes the pains are more sharp and throbbing. I have also been wondering if this is from my back or from kidney. USUALLY, if it is the kidney, the area on the left flank will be very tender when palpated. It is quite possible that this pain is coming from your spine (referred pain). The fact that you have pain sitting to standing seems more like back pain. But, the only way to know for sure is to see a doctor, especially when it comes to your kidneys