You solved this one yourself. You could say she is wearing her swimwear.1)she is wearing her swimming kit (I don't want to specify which kind of swimwear)
I am not an avid swimmer and so dont use technical terms. I would refer to both briefs and shorts as trunks if they were used for swimming.
If a woman said her bathing suit, I would envision a one piece suit, but wouldn't be surprised if she turned up in a bikini.
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