In my dreams when im being haunted why do i go in slow motion?

In my dreams whenever im being haunted or see something that freaks me out i all ways end up reacting to it in slow motion and my actions are really clumsy. For example in one dream i am sitting down at a table with my family and i turn to the corridor and see a really tall girl in a night dress with long hair and she is just standing there. I then picked up a gl to throw at her (don't ask why) but i suddenly went in slow motion and ended up stumbling and throwing it straight at the floor. The ghost or whatever it was meant to be then went into another room in my house and we went to look in there and she had gone. But the slow motion only ever happens when i have a dream that scares me or freaks me out, Ive experienced these dreams since i was about 9 or 10 and they have been happening on and off (Probably with stress) . If anybody knows why please reply asap!