I noticed from your profile that you are almost 60 which is about the age that the first signs of Alzheimer's start to appear. My grandma is 87 now but several years ago when she first started experiencing the symptoms we would be talking and all of a sudden she would look around and not know where she was. I remember one time she started telling me a story about someone she knew when she was in her 30's and when I asked her what year she thought it was she said "Isn't it 1956?"

I don't know much about dissociative disorders so it could possibly be that. Just what you describe sounds like what my grandma went through when she started having signs of Alzheimer's and 60 is about the age they start.