Perfectly normal to me.
You will be surprised how much your body will move on its own while your practice Trekcho. This is a kind of purification.
So don't worry and keep going until your reach the point of freedom from self.
Rely on your insights, they have been a tremendous help to me. Sometimes, you will need knowledge from others like what you are doing right now.
You are doing great. Wish you all the best.
Be careful, you are coming across as someone who is qualified to offer advice on Trekcho, which I'm sure you are not (I come to this conclusion by reading your various comments on Trekcho)
I mean this with respect, but it is very
easy to shift from the role of aspiring practitioner to someone who thinks their limited practice experience has allowed them to dispense advice to others. This can cause obstacles and false expectations for beginners. This is why in Dzogchen and Mahamudra, we rely on the Masters (or those appointed by them). Not people who have read a few books, attended a couple of retreats and attached huge importance to their 'experiences'.
What you're experiencing pretty much sounds to me like what is referred to as a hypnic jerk, which is a common experience while falling asleep. I too have jolted and let out a sound or yelp with this, be it meditating, lying in bed, or falling asleep on a long plane ride. Not saying it's a good or bad thing, and maybe as your meditation gets deeper it is in someway giving your brain/nervous system some signals that it is about to fall asleep.
If this is what you are experiencing, then my own Guru (who is qualified to offer advice) remarked to me that this is associated with the 'Dissolution of the Elements' experienced at death or simply the point of falling asleep. I got/get this very often, he advised that it was nothing to worry about, and gave me instructions. With practice this can be a valuable moment.