Ive a lump in the center of my lower back, just under the skin. Its pea sized and quite hard but not painful at all. I had the same about 2 years ago but it eventually ruptured and some puss and blood weeped out for a few days. It healed up and i thought no more about it - although it left a slight hollow. This feels like the same thing back again as its in the exact same location. Any idea what it could be and if its a concern?