Severe lower back pain after standing or walking for 30 minutes or so?

I am a 28 year old female, 175 lbs, 5' 9" in pretty good shape. This past October I was sitting down in a chair and heard a pop and had excrutiating lower back pain. I had an x-ray done with nothing found. Ever since this day I cannot stand or walk too long (like 30 minutes) or my lower back locks up and I am back in severe pain. If I sit for about 10 minutes the pain dissipates. The pain is shooting down the front AND back of my legs to just above the knee and there is also shooting nerve like pain in my hips when these episodes occur any suggestions?