Possible to get pregnant with IUD?

Hi all! I have had the Mirena for a little over a year now. I know there is like a 99.8% chance you can’t get pregnant with an IUD. I had my “period”/sporting halfway through the month, but 2 weeks later (this morning) I woke up with abdominal pain and very heavy “spotting”. This is very odd because normally I only get my period every 3 months or so. Does anybody have advice on why this happened/experience with the Mirena?

Thanks!!