What is causing my chronic lower back pain?

[BeaNV]

I have had low back pain intermittently for about 7 years. It is a sharp stabbing pain in my lower back that hurts my buttock, and upper leg.  It affects both legs (not at the same time), but the left leg is more affected. I do not have the pain all the time, but have it every day at some point during the day. Nothing in particular seems to bring it on. Sometimes I am sitting, lying down, or standing.  It happens most at work when I am on my feet. When I stretch and touch my toes, it is greatly relieved, but immediately returns upon ending the stretch. The pain is often very strong, and sometimes very sudden and unexpected, causing me to gasp in pain when it first comes on. Most of the time, I can still walk or do normal activity, but with a lot of pain, or I can limp around, but the pain is still there. A few times, the pain was so intense, I could not put weight on that foot, so I could not move. This has only happened twice, and was relieved to normal pain within a minute. I did see a chiropractor, which did not help. They dis and X-ray, and the discs and spine looked normal. It feels much like how sciatica sounds, but a little different. It has been going on for a long time, but I have had it more frequently lately. Sometimes the pain lasts for an hour, sometimes all day. I have never seen the doctor for it, as I assumed back pain is normal for an aging person (37).  Everyone I know, though says they do not experience this.  Any help you can give is greatly appreciated. Thank you.