I just realized I have something similar to the internal tremors sometimes mentioned on the forum. When I have really bad days, my symptoms are the worst right after waking up and I have two symptoms that are only present at this morning time but they also disappear relatively quickly. One of these is not being able to hold my balance well at all, the other is this vibrating feeling. The vibration is not a new symptom, I've had it several times at my worst but I have never paid too much attention to it as it is short-lasting, somewhat rare and not really bothersome to me. I keep forgetting about it.
It is a vibration, not intense but noticeable. I don't shake or feel any kind of actual movement and it can be felt deep inside my body, in my trunk/upper body. I have no idea what the reason can be. I don't know what it is that actually vibrates inside my body.
I'm not severe, so this may be a milder version of what some other people experience (similarly to how my sound sensitivity is significantly milder than for severe people). Or it may be a different thing, who knows.
