She didn't hold her breath. A lot of girls don't have naturally flat stomachs, they just suck in all the time. Its the same for when youre laying on your side. So some girls suck in when they're being spooned too, so that the boy doesnt feel how big their stomach really is. But it gets tiring after a while, which is why the redhead in the cartoon moved his hand to her boobs. She wanted a break from sucking in.