I answered on the ask Lemmy round of this question but - lack of desire. When I was getting periods, the premenstrual time was the highest desire, period the lowest, and oddly enough, ovulation second lowest. The days right before the period, so intense, everything felt so much and so good, then a sharp and sudden drop, no desire, migraine headache and sex not an attractive idea. The sharp drop in hormones that let ovulation and menstruation happen were always accompanied by a lack of desire for sex, for me.

Physically the cervix is at its lowest point too, and hitting that can cause cramping, but mostly it was just the absolute not wanting to.