I noticed this tonight and I'm not sure what the hell the cause it. My upper back is flat as a board and so is my lower back until I get to parallel. At that point it rounds and it seems like ****** dips down underneath. I doubt its flexibility as I do yoga and have hypermoble joints. I can't seem to keep it from happening. Anyone get this or know how I can fix it (assuming its fixable and not just my particular anatomy)?