Nearly every time I do low D, C sharp, and C from the Overtone Series, my lips/muscles start trembling. Even after only 30 seconds of playing. Why is this?

it might be genetic (no joke, one of my really good clarinet friends has this problem) or...do you drink a lot of coffee? i shake when i have any caffeine so that might be a reason...

I`m not sure if it is genetic; no one in my family plays the flute except me. And no, I do drink decaffinated tea but never coffee.

"I like coffee, you like tea. That`s why we don`t agree!"

then the only thing that i can suggest is that you work on developing those muscles. talk with your teacher about it best of luck