ScanScan:
Don't discount a disc problem just because you have no pain in your back. What happens is the disc can become prolapsed or herniated and basically bulges out from between the vertebrae and presses on the nerves which run down your spine, especially the sciatic nerve which runs right down your leg.
I too had lower buttock and leg pain, subsequent MRI scan confirmed the 4/5 vertebrae disc.
I have suffered on and off for years and during the last bout ended up having traction. Fingers crossed I have been clear now for over a year - the longest ever!!
Best of luck - I know just how painful it can be.