Suggest Treatment For Prolonged Muscle Spasms On Lower Back

I have been experiencing prolonged muscle spams on my low back lasting more than 4 months. The last week was absolutely severe i.e as I was not able to walk. I experience absolutely no pain when I am working out in the gym(light weight training) or doing YOGA or other forms of stretching. But, when I walk, the muscle in the lumbar region tends to get tight and I come to a halt. All that it takes is 30 seconds of walking to experience pain. I have been taking Etoricoxib 90mg and Thiocolchicoside 8mg for the past week, but the medicine is not working the way it used to. I am able to do forward leg raises for 90 degrees, knee to chest stretches, flexion and extension, and YOGA poses with no pain at all. Only walking induces this pain. Do I need more powerful medicines or is this some other problem that needs more urgent attention?