SYSTEM REFERENCE DOCUMENT 3.5
Certain effects give a character temporary hit points. When a character gains temporary hit points, note his current hit point total. When the temporary hit points go away the character’s hit points drop to his current hit point total. If the character’s hit points are below his current hit point total at that time, all the temporary hit points have already been lost and the character’s hit point total does not drop further.
When temporary hit points are lost, they cannot be restored as real hit points can be, even by magic.
Increases in Constitution Score and Current Hit Points: An increase in a character’s Constitution score, even a temporary one, can give her more hit points (an effective hit point increase), but these are not temporary hit points. They can be restored and they are not lost first as temporary hit points are.