I would imagine there's different triggers for everyone, but what jumped out at me was "bathing suit" and "slept with a washcloth between my legs."
Bathing suits used to always give me yeast infections -- not so much anymore. And sleeping with anything moist between my legs would absolutely give me one, even today. Even my doctors tells me the drier the better down there, particularly when I sleep.
So I don't know about sand, but I would say a bathing suit and lots of moisture over a period of hours in a dark area (between your legs) sounds to have the right ingredients for a healthy dose of yeast.
Hope it clears up for you quick