Among the most common causes of chronic pain is low back pain. Recent study data indicates a "strikingly high" correlation between chronic low back pain and Ankylosing Spondylitis. In my case, I had unusual wear on the low back due to a severe knee injury which caused a limp for a number of years - a limp where the low back and hip took on the load bearing for the injured knee, as well as a shearing motion in doing so. This strain could have been a contributor to chronic low back pain. Then there was the osteoarthritis which was more advanced in me than typical in young people as a result of the injury and surgeries such as bone-grafting and other highly invasive interventions. Then there were episodes of lumbar disc herniation requiring surgery. All of these alone or as a group, as well as a chronic pain syndrome wherein the overstimulation of certain neural pathways causes perceived pain in the low back, could be responsible for chronic low back pain in my case (and at least some of these perhaps in your case). Since it on average requires 7-10 years to diagnose AS, and since AS is most successfully treated early in its' development, there is a new push to give primary care physicians better information so that that long period where AS is seldom diagnosed can be brought down to a shorter interval - and hopefully therefore, allow more effective treatment.
Not only does AS turn out to be present in over twenty percent of patients experiencing chronic low back pain, but that number may be significantly under-reported. This abstract, and the full article are worth reading if you, or someone you care about is experiencing chronic low back pain.
Remember this: Just because there is something that MIGHT be causing chronic low back pain (injury, prior surgery, wear and tear arthritis, etc.), don't rely on the fact that your chronic pain COULD be due to one of these other causes, get to an experienced rheumatologist who will carefully look at your status and make a diagnosis of AS if appropriate. In my own case, the first rheumatologist I saw, about four years into the progression of symptoms, noted that I had injury and osteoarthritis and made the assumption that these were the cause. My own outcome from treatment of AS would likely have been far more successful at early days in the development of the disease, than it has been since I saw an excellent rheumatologist about eight years into the situation. If you are referred to a rheumatologist, or have endocrine issues accompanying your chronic lower back pain, see the rheumatologist and by all means get a second opinion. Four or five years lost in fighting AS is a damn shame.