Interesting because I have experienced the same thing at times - what felt like tremors, but only in my upper body. It has only happened to me while laying down on my back and while sleeping on my back. Once I woke up and started to move around, it stopped. Scary because I didn't know what was going on and thought maybe my heart was racing, but when I felt my pulse, it seemed normal. I have described the same way as you, feeling like my insides were shivering. I also had (and still have) cervical spine issues including a large disc herniation at C5-6, cord compression, and smaller herniations and stenosis at other levels.