。
She was sunbathing at the time.
Literal
She [topic-は] at-that-time sun-bathing [object-を] was-doing.
日光浴 ('sunbathing') is a tidy three-kanji compound: 日光 ('sunlight') + 浴 ('bathing'). Japanese builds many activity-words this way: 海水浴 ('sea bathing / swimming at the beach'), 森林浴 ('forest bathing'). The ~浴 ('bath in X') treats sunlight, ocean, and forest air as substances one bathes in — a notably contemplative framing that fed into the global popularity of forest bathing as a wellness concept. The polite ~ました fits a written report or polite spoken account.