Have you ever felt like there was something wrong but you don't know exactly what?
Like your body is trying to tell you that there's something not quite right, but if you were to go to a doctor, you wouldn't be able to give them an exact location of pain or discomfort, just an overall sense of something being out of wack.
That worries you and so you start thinking of all sorts of things that could be wrong, which probably aren't whatever it is if there really is anything wrong at all.
Another example is when you feel like there's something big about to happen somewhere, somehow, but you have no idea what that thing is, when or where it will happen. And you don't know if it's a good thing or a bad thing.