Th only thing I can think is it happened at around the time he started following her on Twitter - but that doesn't work as he was still in Vietnam.
I think it actually was at the Met Gala. She was still with CH then, even if there had been trouble in paradise that nobody knew of - so a one night stand allegation would have hurt her. And the perfect Disney prince. She then went on a Caribbean holiday with CH in May - hence the sneaking away while in a hotel lyric. Possibly she went because she wanted to try to make it work with him after the one night stand.