If you feel pain from your lower back to behind the thighs spreading down below your knees, you might be suffering from sciatica. Sciatica can be understood as the pain and sometimes numbness as well as weakness which spreads along the sciatic nerve.
The sciatic nerve is located deep in the buttock. Because of its vicinity to the piriformis muscle, constriction or swelling of the muscle may lead to irritation of the nerve and pain. (Psoas Trigger Points Lower Backs)