Lower back spasms can cause excruciating pain.  A doctor explained it to me this way–When a muscle spasm starts in your back, the natural reaction of your muscles is to tighten to protect your spine.  This tightening can cause more spams which cause more pain and the cycle repeats itself.

Once a cycle of back muscle spasms starts, it is difficult to break that cycle.  To stop the cycle will usually involve muscle relaxants and bed rest for a few days.