The same as hers. Imagination.
She claimed to have been wearing a bathing suit under her clothes but made no mention about the party being at a house with a swimming pool or hot tub. Details that would have been relevant.
The bathing suit actually sounds like the kind of thing one wouldn’t think to invent. Perhaps she was once groped at a party to which she’d gone after swimming.
Interesting that she recalls wearing a bathing suit and now it is said she wore it under her clothes. Did she say it it was a one piece or two piece? I’m thinking a one piece would be more difficult to remove, especially under clothing, then either underpants or a bikini bottom. The question is what kind of clothing was she wearing? Pants or shorts? A dress or skirt? Does she remember?