Imo one more thing is that when you're in a relationship, you should want to know as much as possible about your partner's health, about their symptoms, the conditions they have... I asked my partner if I could see which medicine they take, so I'd be aware if there are bad secondary effects I could spot and warn them about.

When you are on your periods, your cervix might be more opened than usual. I remember il r/badwomensanatomy someone posted a "good anatomy" serie of pictures she took of her own cervix during periods, it was instructive. That's also the reason why doctors tell you to get an appointment during your period to put the iud in. That said the pain is different for each person.