Why do I feel like I'm dreaming and dream things that happen?

Sometimes out of nowhere I will feel like I'm in a dream. By this I mean I don't feel things as much as I should like pain etc. I will feel a bit detached and my mind sometimes wander or goes blank so I have a hard time paying attention. Then I start to think I'm really dreaming sometimes so I pinch myself. Also, for a few year I will dream something and then it will happen sometimes it is the next day sometimes it's 9 months later. I know for a fact I really dreamed it I'm not crazy. It feels like de ja vu ( or however that is spelt). And whenever I feel like I'm dreaming it is usually triggered by a sense of de ja vu. Thank you.