If I understand your question properly, you want to have an element fixed to the bottom of the window if it would usually be further down the page and out of view. And when the user scrolled down to it's natural position it would flow with the scroll as normal.
I modified your function slightly to remember the elements initial position on page load and use that to compare it to the scrollTop position each time.
$(function() {
var $postQ = $(".show_postQuestion"),
$postQPos = $postQ.offset().top + $postQ.height(),
$win = $(window);
$win.scroll(function() {
if ($postQPos > $win.scrollTop() + $win.height()) {
// Post form off-screen
$('.show_postQuestion').addClass('fixed');
} else {
$('.show_postQuestion').removeClass('fixed');
}
}).trigger('scroll'); // trigger the event so it moves into position on page load
});