Maybe it's not kidney but back pain from your gallbladder or even a hiatal hernia. Ask the medic to order an addominal ultrasound. This usually shows any stones and is the first step in defining the problem. Also, they may want to order a HIDA/CCK to more fully evaluate your gall bladder. The symptoms you have -- gastric reflux (the acidic taste), gassiness and mid to lower back pain is often associated with gastric problems.
You might want to look into the Antacid/Candida Overgrowth vicious cycle, especially if you've ever taken antibiotics. I recently kicked my antacid habit of ten years by following an anti-candida diet. Alkalinizing the intestinal track (with antacids) allows candida to grow and morph into its fungal form... In the last year, I often took a Tums not to stop burning in the throat/esophagus, but to stop pain in my right kidney. I'm still figuring it out. Just tonight I ate some raw tomatoes and got the kidney pain big time. From what I've researched, it could be the solanine, the acid, or the potassium in the tomatoes. I'm convinced I have debris (i.e. tiny stones) in my kidneys and have been drinking lots of water and using kidney cleanse supplements. They are helping. I drank lots of caffeine and little water for many years... How are you doing?