the trouble with back pain is that many injuries of the lower back present in much the same way. A recent science paper showed that approx. 50% of people surveyed had a herniated or bulging disc, most of which was asymptomatic.
it could be a slipped or bulged disc that is causing the pain... but it could also be a torn muscle, or it could be a locked facet joint(s) from chronic stiffness in the lower back, or a combination of several of these.
At a guess (and note I not a chiro/physio) it sounds to me like you hurt some musculature in your lower back and the cramping and stiffness perhaps locking a few facet joints thereby limiting any kind of extension or twisting.
My hope is you know a good chiro!!! I would ask for referrals from other guys who lift.