Sometimes it's definitely easier just to say
but those in the know know that you are so far from fine and although they don't know the precise details of the perfect storm of emotion raging inside you, they know not to ask more, they know not to delve but instead to back off and give some distance, knowing that you will let them in when the time is right, when you are able to talk and give more. For the moment, however, that "I'm fine..." might as well be a "Leave me alone!" or a "Don't ask!" or even a "F**K OFF!".
So remember this: if you have a friend who says "I'm fine..." and you are worried about them; be a friend, check back on them - just in case - and let them know you are there for them: I promise you, you won't be turned away.
ps... this is just like saying "i'm tired..." when really you are just a little sad...