The first section follows a simple negative statement. While making a negative statement, you should always shake your head for most of the sentence.
The second section follows a TOPIC+COMMENT structure (with “hungry” being the topic and “I’m not” being the comment). Notice how Zoée raises her eyebrows while signing HUNGRY, this is how she establishes the topic, then she follows up with the comment.
Another way you could say this is to sign HUNGRY ME while shaking your head. This would be a casual way of saying that you’re not hungry.