May be taking things out of the general train of thought here, but having someone check your pelvic stabilisers/hip muscles may be a good idea, Bushy touched on pain being reffered from an area into your lower back.
I am currently familiar with this situation though i do know how mine began.
Bad posture has been mentioned and in myu experience, the general gist amongst lifters is poor development of the spinal errectors and lack of flexibility in the lower back, poor flexibility and length of the hamstrings, leading to more pressure on the hips where the weaker muscles such as the piriformis take too much strain and cannot cope with it i.e dead lifting with 80% of the load transfered to your hips!
3.5 tonnes hit in me in the chest and a wall fell on me which was the cause of my problems but i have spent perhaps 200 hours now researching various rehabilition programs for the hips, hamstrings, quads and lowerback which would lead me to believe this is perhaps a more common problem than people think.
Then again you could have fucked up an inj and ruptured a nerve, transference into the LB??
So many possibilities.