I'd be more concerned for her health than getting bumped. Twitching is quite common. Violent jerks suggest a neurological dysfunction. It may be worth checking with a movement disorder specialist.

If her head becomes ballistic (continues to move after the start of the twitch) and moves more than an inch from its resting position, it probably should be checked.

I used to have twitches where my arm or leg would fly across my body and smack my wife when I was just trying to move to roll over. It was a motor neuron issue.

Head movement is rare and usually is not the primary movement. The body may jerk causing the head to move. This is not uncommon at the transition from awake to sleep.

I bet this is more of an anxiety issue than a risk. If you are going to cuddle, just don't put your head where her head and impact it. Her head on your shoulder would only bump your chin/jaw.

Plus, from an impact force perspective, there may be enough impact to be uncomfortable but it is highly unlikely the impact for would get anywhere near a concussive force.