。
She was happy one moment, sad the next.
Literal
She [topic-は] rejoicing [or-たり] grieving [or-たり] did.
~たり~たりする is Japanese's 'list a few representative actions' construction, signaling that the named events are samples from a larger range — not exhaustive, not necessarily simultaneous, often alternating. The form attaches to the past stem of verbs (喜んだ→喜んだり, 悲しんだ→悲しんだり) and closes with する as a generalized predicate. Here the alternating contrast between joy and sorrow gives a portrait of emotional turbulence — the same pattern would apply to 食べたり飲んだり ('eating and drinking,' among other things) or 笑ったり泣いたり ('laughing and crying').