I just had my 2nd mirena inserted and alos have very sore nipples. I can't remember having this side effect when the 1st mirena was inserted.
Her doctor probably didn a pregnancy test before inserting the Mirena. Anything is possible, but it could just be her body adjusting to the IUD and there is a little hormone in Mirena so that could be part of it as well. she can always go on their website to look at side effects.