Can you explain my Deja Vu?

Sometimes when my eyes are observing something, (such as a teacher explaining something to me, a person holding a sign up, a game I'm playing, etc), I get an eerie feeling of Deja Vu. There're only mere, short scenes, but they feel as if I'd had a... dream about them before. I can't explain it. Then, when that event actually happens, I feel my eyes widen, as if I'm not even looking through them anymore.