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.

Not sure why your getting downvoted just chiming in with your own experience. Although your post history is a bit questionable...