。
She happened to be in the bath.
Literal
She [topic-は] by-chance bathing-mid was.
入浴中 is a Sino-Japanese compound: 入浴 ('bathing,' literally 'enter + bathe') plus the suffix 中 ('middle, during, in the midst of'). The 中 suffix is a productive way to mark an ongoing situation: 会議中 ('in a meeting'), 食事中 ('having a meal'), 工事中 ('under construction'). It feels more written and slightly clinical compared to the conversational お風呂に入っている. Bathing in Japan is a serious daily ritual — typically a long soak in hot water lasting 20–30 minutes or more — so 入浴中 marks a state visibly inconvenient to interrupt.