"Did old people ever tell you not to sit on rocks etc because it'll make your uterus cold and infertile? I wonder if the myth comes from here."

LoL, that has to be an old wives tale...🥶❄ 

 Yeah, and a very common one too, across cultures. Don't sit on the floor, etc. It's bullshit ofc but the cold hurts so much on my period it feels like a big deal lol.