Actually, I know exactly what KingYoshi is talking about when he talks of this feeling that it is a dream, even though rationally you may not have quite realized it yet. It's like you have noticed stuff, which adds up to it being a dream, but you have not added up all the parts yet, and so if someone asked you why did you know it was a dream when you did, you might not be able to explain it, but of course there were reasons because you noticed stuff that only made sense as a dream.