I do that and I don't have a dissociative disorder.It could just be that you forgot the movie or maybe when you watched it before you were distracted by other things and didn't pay close attention to it.Lots of times I get phone calls or go online while I'm watching movies and don't really get engrossed in them.

There's so many movies that I watch and realize I've only seen bits and pieces of them and not the entire thing so it's like watching them for the first time really.