Posted: Tue Feb 20, 2007 12:18 pm
It looks like it could be an abcess to me as well. An abcess is an infection that manifests itself in the form of a lump (think of an abcessed tooth). Pus builds up and the more pus, the bigger the lump becomes; as it grows it puts pressure on surrounding tissue and becomes increasingly painful (but your turtle can't tell you this). It won't go away by itself---it should be lanced open and drained (the procedure isn't complicated). Abcesses are often located on the sides of the head (where the "ears" are) but can appear on other areas as well. Antibiotics are often prescribed to eliminate the infection if it is systemic.
As for the cause---somehow bacteria has breached the outer surface (shell, skin) and caused the infection. Not keeping the water clean enough is one factor that can contribute to this condition.
"You become responsible, forever, for what you have tamed." -Antoine de Saint Exupery-