The reason it feels wrong is that “are” is the main verb in the sentence and shouldn’t be contracted. You are only supposed to contract auxiliary verbs like “you’re eating already” where eating is the main verb and are is auxiliary.
Edit: (I used a bad example because “eating” is a noun, as pointed out below.)
Un-edit: The example’s correct, “eating” is a verb in this context.
Also, I’m thoroughly confused about who’s saying “you’re already” in this comic.