My understanding of feeling "flat" is a general lack of any emotion whatsoever. Not sad, not angry, not happy, not anything, despite any outside stimulus. If you're flat you're blank.
Co-Moderator, Bipolar Forum
It is a melancholy of mine own, compounded of many simples, extracted from many objects, and indeed the sundry contemplation of my travels, in which my often rumination wraps me in a most humorous sadness. -- William Shakespeare