Solved !!!

PVector mouseV() {
  PMatrix2D m=(PMatrix2D)getMatrix();
  m.invert();
  PVector u=new PVector(mouseX,mouseY);
  PVector v=new PVector(0,0);
  m.mult(u,v);
  return v; 
}

float mousex(){return mouseV().x;}
float mousey(){return mouseV().y;}