It's probably just fatigued
Stretch it gently and hold the position, repeat as often as needed.
There is not a single muscle that rotates the upper arm. The name of the muscle that contributes in rotation of the upper arm is deltoid muscle.
Skeletal muscle is associated with the upper arm.
The brachialis is a muscle in the upper arm that flexes the elbow joint.
What are the tendon in your arm?
no
I believe when your lips are "jumping", that you're just having a muscle spasm.
Biceps or triceps
where you bored when you posted this question or have you pop you muscle in you left arm
Deltoid
which shoulder muscle allows you to adduct your arm
It depends. But, the triceps are 2/3 of your upper arm.