What Could Bruise On Back Indicate?

Hi. I have a bruise on my back (lower spine) and its been there for a few years. I cant recall exactly how I got it, or if it was even caused from an injury. But because its been there for so long, I'm extremely concerned. I'm trying to get to my M.D but I work and the only time I can get there is late afternoon or weekends, so it may be a few days til I can get there. Do you know what it could be?