@Technez I'm responding weeks late, but honestly, same? It's a confusing turn of phrase and if I remember correctly I couldn't really find anything more elucidating online about that being like, a thing people say.
My interpretation is that it's like a swimsuit in that it's something you just slip into when it's convenient. Or something like that. Maybe in that swimsuits are considered attractive, as in she's trying to make her like her more? I really don't know.
Though looking back, I could have done better on the phrasing. To be precise, she says something like "like she's turning my feelings of liking [things] into a swimsuit," and I left parts of it out...not that it's any clearer even then. I'll probably just reword it.