Drawing GUI rectangle uses negative y coordinate

I have a function to draw a healthbar on top of my players, and I'm using WorldToScreenPoint to draw it just above them. It draws the healthbar at the appropriate x coordinate, but somehow it's drawing it at the opposite y coordinate. For example if my player is at a y of 1, the healthbar will be at -1, and as the player's y rises, the healthbar's y lowers. Here is the code I use:

 function OnGUI()
 {
 
     var screenPos : Vector3 = Camera.main.WorldToScreenPoint (transform.position);
  
     // draw the background:
     GUI.BeginGroup (new Rect (screenPos.x, screenPos.y, size.x, size.y));
         GUI.Box (Rect (0,0, size.x, size.y),healthBarEmpty);
  
         // draw the filled-in part:
         GUI.BeginGroup (new Rect (0, 0, (size.x * health) / 100, size.y));
             GUI.Box (Rect (0,0, size.x, size.y), healthBarFull);
         GUI.EndGroup ();
  
     GUI.EndGroup ();
  
 } 
 

Also, using the negative of the y position seems to just make the GUI disappear altogether. Anyone see what's wrong?