Duster would have that queasy/nauseated feeling inside him and be quivering gently because his body is failing to comprehend he shrunk down. Not only that his body is reacting negatively to being suddenly forced into a very tiny form with little chance to process it so he’s feeling like he’s getting sick without actually having to run a fever or abnormal temperatures. His body isn’t okay, because he isn’t okay; he’s emotionally overwhelmed as well because everything is literally towering above him and he can’t fit or wrap his head around it as easily as he could if he was just looking up at a tall building
“Buildings are made to be tall, but this isn’t. I’m not made to be small--this isn’t how it’s supposed to be.” Duster would be aware he’s freaking out and feeling sick because of his situation--it’s natural to freak the fuck out. You’re literally the size of a thumb and everything you knew about the world was sort of just swiped away instantly. But Duster is an emotional human being, and possibly after experiencing something near death through being so small and easily unnoticed he’d burst into tears from utter fear and anxious thought scenarios in his mind how he can be killed, stretched apart, eaten by insects, or squished.
Then Duster would start thinking himself pathetic; he tells himself he’s an appropriate size for someone who’s a “moron” and “can’t help but get himself into trouble”. He thinks, like an insect, he’s just a pest to his friends but now he’s physically considered a pest to some people because he’s smaller than a rodent. He begins to chew on the idea that he should accept his small size and perhaps allow himself to be eaten by some bird or stepped on by a person, but of course he dismisses it and thinks to himself how stupid that sounds, knowing his friends are probably worried beyond belief and searching for him. It be his job to get their attention and maybe there’s a chance he can return to normal. That’s when Duster would decide to take action.