Deimos,
I am right there with you. The shaking came with the released anger that I've held for 30 years toward my abuser. Then, when any significant anger surfaces, I shake some more and sometimes feel cold, too. As you said, therapy brings it all up to the surface, and the body will react to it. Why not? It's waited a long time to have it's say.
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Trauma therapist Peter Levine has written about this as a release of the fight or flight energy that was held in our cells at the time of what happened. When we freeze for our own safety, we're holding anger (fight) or fear (flight) energy ready to use at any opportunity. Then when the danger is over, what should happen naturally is we shake straight away and discharge it. But with trauma, we keep holding it frozen, and over time as it tries to unfreeze/surface and we push it back down we refreeze it so it becomes even more entrenched.
If you're shaking, you're actually releasing some of that energy and therefore some of the trauma that your body has been holding. Often, you can feel cold as well, sometimes itchy.
I have a somatic therapy (craniosacral therapy) that works gently with this principle, and for the first few months I shook for hours some days. I would also get extremely cold, especially on the soles of my feet. (I've had severe trauma which had been frozen for years due to amnesia.) It was weird but I was so glad to feel that trauma energy leaving me. The more that happened my PTSD symptoms got less severe, especially the anxiety (I'd been holding a lot of frozen fear and I think what the shaking was releasing was related to that).
Peter Levine explains it in his book "Waking the Tiger: Healing Trauma".