When the discs between the vertebrae bulge they don't have to bulge very far before they start pinching a nerve. When the nerve gets pinched there will some inflammation which causes stiffness and further pinching and more pain and more inflammation. They will inject some steroid into the epidural space around the spinal cord. After the first day or two, the steroid should help reduce the inflammation and decrease the pinching and hopefully stop the cycle. Also, hopefully the disk will heal and stop bulging and pinching the nerve which would then relieve your pain.
Be sure to take smaller doses of ibuprofen with food more often rather than taking the big doses less frequently. Also don't take it and then go lay down flat immediately. That gives it a chance to cause irritation.