There are nightmares i had as a child, and though they were a confusing jumble of images they were terrifying. When I have tried to describe them to people they sound almost comical in their banality, yet recalling them makes my hair stand up on end. I wonder if there is some element in your feelings about certain images that are similar?
Let me tell you about my nightmare and some of the things that bring it to mind. See if there's some element that is familiar about it. The first time I had the nightmare I was about 10 or 11 yeas old. My family and I were in a cabin in the mountains and I think I must have woken up the entire campground when I screamed. My nightmare, at least what I can remember of it, consisted of large stone and iron balls, like cannon balls or bowling balls falling from the sky and landing on very delicate needles sticking up from the ground. They would, of course, crush the tiny needles and they would make a deafening noise when they struck the ground. Now the thing that stuck in my mind about this nightmare was the difference in scale of the two objects. The ball was massive and large, the needles were tiny and delicate. I had similar nightmares two more times, but the details are lost. The similarity with the first was that same contrast of sizes. Just thinking about this aspect of it actually gives me a feeling of queasiness. Many years later, I was reading a geology text book and there was a passage describing the way that earth scientists categorized rock sizes. The book explained that they used a scale with boulders on one end and it progressed all the way down to the finest silt at the other end. That passage brought the sense of dread and terror back for a second and I almost slammed the book shut. It passed, but even now, 20 years later, and 40 years after that first nightmare , I can still feel a twinge of that same horror I felt as a kid. It sounds as if the feeling you describe is very much the same as what I've felt. It is such an overwhelming sense of dread.