The doctor diagnosed it as a pinched sciatic nerve. I also suggest that you start a regular stretching program. The American College of Sports Medicine recommends stretching three times per week. Increase your body temperature first, then hold each stretch for 18 seconds. There is quite a vast amount of research on muscle balance and the problems and injuries associated with imbalances.
Imbalances lend themselves to injury, and pain is often the result.
A common muscle imbalance called hyperlordosis results from tight lower back muscles and weak abdominal and trunk muscles. This condition is very common. When people complain of occasional low-back pain, pelvic tilts are the exercise I recommend. To perform them, stand upright, with feet hip-width apart and knees slightly bent. Place your hands on your hips and flatten the lower back by pulling your belly button closer to your ribs.