When most people say "I've lost my appetite" they've usually come across some shocking bit of news or a realization. Or they are simply too squeamish to handle such gruesome conversation. At least, that was sort of the case for me as a child.
I remember a Halloween night topic of body parts and dismal ends. I claimed to have "lost my appetite" although I continued to eat.
I've never physically lost my appetite But maybe I haven't had the proper shock.