。。
She looks pale — she might have been sick.
Literal
She [topic-は] complexion [subject-が] bad. Was-sick [might-かもしれない].
顔色が悪い ('face-color is bad') is the standard idiomatic way to say someone looks pale or unwell — 顔色 reads as both literal complexion and a barometer of health and mood. The related expression 顔色をうかがう ('to read someone's face') extends this idea to social attentiveness, even sycophancy. The second clause stacks 病気だ (the noun-predicate 'is sick') in past form and adds かもしれない to give 'might have been sick.' Choosing the past 病気だった rather than present 病気 signals reasoning backward from the visible evidence (paleness now) to a prior cause — a classic past-speculation use of かもしれない.