Your condition is known as spondylolisthesis. In essence, one of your vertebrae has slipped forward on the one below it.
Stress fracture occurs when a bone is repeatedly placed under too much physical stress.
You can also develop it without trauma. When it fractures, it allows the front of the vertebrae to move forward away from the rest of it. Best to look for a vertebrae in a book to see what I mean. And it doesn't mean you're doomed to low back pain all the time, although some people certainly have more problems than others. Easy to get it moving by getting it manipulated by a chiropractor or osteopath.