Hi,
I have had very strong feelings of this and did not know what was going on.
It happens more readily for me when I am feeling at my low points and I have been to the cinema. If it is a good (believable and with strong emotions) movie then I will leave feeling immensely disjointed. The world about does not feel at all real as being in the cinema I became part of the movie and feel joined within it. Once the movie finishes I can have quite a hard time believing that the real world is not just an odd dream I'm having as my reality has partially shifted into the movie. I wouldn't say that I was completely out of control as I am aware of my faculties and know what I'm doing, but my mind swings in and out of which is real and which is not. It doesn't happen every time but does appear to have a strong emotional trigger (great loss or sadness) which some movies are very good at bringing on especially when the cinematography is well done. It doesn't usually last all that long (maybe an hour) but when I'm alone it does appear to last much longer (which can be a long way into the evening and has once been overnight). When I'm with someone I know, or meet people I know soon after, the feeling disappears much more quickly, which is probably due to the fact that they help me to join reality much quicker.
I suppose this maybe more of an identity thing, but it is definitely derealisation as well.
