Not Deja Vu

I've been experiencing something odd lately. It reminds me of deja vu, although more like the opposite.

I've experienced deja vu before. It is an odd sensation and likely something to do with our brain's ability to connect things (to quote Doctor Who, "I love humans. Always seeing patterns in things that aren't there.").

What has been happening to me lately is that I'll experience something as wholly novel, but then later begin to feel like I had experienced it before. I've really begun to doubt myself as I am as sure that something seems familiar later as it does not seem familiar to begin with.

One of the latest examples is a friend's picture on FB, where someone took a Revolutionary War scene and replaced all the weapons with lightsabers. Oh, and there's a Death Star I ddn't notice until just now.I saw this the other day and thought, "That's so interesting." I noticed that the friend had uploaded it in 2012 and wondered how I had missed it.

At some point over the next day or so, I started to think maybe I had seen it earlier. Wasn't it familiar after all?
To really appreciate my unease over all this, you have to understand that I have a really good memory so am not used to forgetting things. I would actually think I had an even better memory than I do were it not for times (and I'm talking maybe two) when I have this conversation with my friend Dan:

Dan: "Remember when ...?"
Me: "No."
Dan: "There was the .."
Me: "No."
Dan: "And we..."
Me: "Oh, yeah."

I also have examples where I just remember things I've totally forgotten about. And then, recently, I went into an old e-mail account to search for account information and found all sorts of things I don't remember doing. So, maybe it is my memory going. Maybe I was under stress when those memories were formed and thus they cannot be accessed as quickly as others. I just know it is annoying and I need to figure out if there is a way to have the sense of familiarity earlier, before I start e-mailing things off and then thinking, "Wait, did I already send them that?"