。
She puffed out her cheeks.
Literal
She [topic-は] cheeks [object-を] puffed-out.
頬をふくらませる is a set expression for showing displeasure by puffing out the cheeks — the quintessential visual of pouting. ふくらませる is the causative of ふくらむ ('to swell'), so literally 'she caused her cheeks to swell.'