I've had hip replacement 8 years ago, and am now experiencing pain in the rear of the hip and down my leg. What would cause this pain?

Sciatic nerve pinch. Based on it's location, the pain is most likely due to compression of the nerve root as it exits spinal canal (at the level l5-s1). The test, that is most revealing, is MRI of the lumbar spine. Consider visit to pain specialist for evaluation and treatement.