Shiver. It immediately evokes a chilling cold feeling. But a shiver is also an involuntary reaction, like when your leg jerks after your knee gets tapped with a hammer, or falling hopelessly in love after seeing Jennifer Lawrence for the first time.
What's that about love? Well, this is also a novel about love, and how it, too, is a seemingly involuntary reaction. With all the talk of scents going on, it has to be because of pheromones.
So although the act of shivering is tied into the constantly fluctuating temperatures, which are noted at the start of every chapter, it also has to do with an emotional reaction to love and loss. Near the end of the book, after losing Sam, Grace remarks, “Even though I knew I was alone, I still shivered” (64.8). In this case, it's not because of cold, but because she's lost Sam. Oh, and she kind of feels like she's being watched. Which she is. That's something else about shivering: sometimes it happens when there's a creeper nearby. Pay attention to what your body is telling you!