Since you did not indicate that the xrays of the spine were abnormal, I will continue in that presumption. Uric Acid is a normal waste product from eating animal fats, gravy, high amounts of oils, and/or processed foods. Normally the kidneys remove uric acid through urination. When Uric Acid is high, it indicates a buildup that could result in kidney stones or gout (arthritis). It can also cause a chronic low state of dehydration in the body. Pain in the lower back can result from kidney problems, chronic low level dehydration, nerve damage, disc problems, poor posture, or sedentary lifestyle. If you walk with your pelvis at an angle which would cause a slight bending of the knees, you have a posture problem that can becausing back pain. If you spend your days on the computer, mostly sitting, or minimal walking, that can cause back pain. If the pain during walking is from dehydration, you can drink 2 large glasses of water while you walk, or upon the onset of pain. If the pain stops within about 15 minutes, it is being caused by dehydration probably caused by the Uric Acid buildup. If your pain is from poor posture, correction of the posture will correct the problem. If the pain is being caused because you just don't walk enough, gradual increasing your walking will improve your pain over time and increase in activity. The exercises your doctor ordered serve to stretch the back and pelvic muscles and reduce your pain. The order of calcium may indicate that your doctor is suspicious of thinning of the bones in the pelvic/hip. My suggestion is to improve you posture, especially when walking, drink more water, take your calcium, continue your exercises, and increase your general activity levels. If you smoke, stop. If you drink alcohol, give it up (forever would be best), at least until your pain subsides. Have your kidneys checked with a test called an IVP, and stay away from high fat animal meat, gravy, and processed foods for a while to give your kidneys a chance to eliminate Uric Acid and heal while you are waiting for the kidney test. Good luck.
I forgot to mention that the pain starts only after a period of 15minutes of standing or walking.
I want the advice from 'star queen', for I've seen it is only 'star queen' that extends most sincere advice.