。
She felt very ill that day.
Literal
She [topic-は] that day very mood [subject-が] was-bad.
気分が悪い is a useful double-meaning idiom: 'feel unwell physically' or 'be in a bad mood,' depending on context. Without further cues, this sentence usually reads as the physical sense — 'felt sick.' The pattern が悪い ('be bad in respect to X') is built into a long list of stative descriptors: 体調が悪い ('be in poor health'), 機嫌が悪い ('be in a bad mood'), 都合が悪い ('be inconvenient'). Note その日 ('that day') as a bare time-frame, no particle — Japanese routinely drops time-に in casual or written contexts.