Many things. Don't want to suggest something to you; you know how anxiety is.
I just saw your other thread, and like you, I have neck pain and inner trembling/tremors on various parts of my body. Even my abdomen and back. I was told that my neck pain and spasms were caused by fibromyalgia, but I also had a physiotherapist tell me that trapped nerves in the neck can cause various symptoms, including tremors/tingling/numbness/pain. Not sure if it goes all the way down to the leg or abdomen though, but I think it'ss omething to consider.