First PP period or IUD issue?

Any moms out there have trouble figuring out if you got your first PP period or if there’s a problem with your IUD? I’m currently breastfeeding my daughter and haven’t had a period since she was born. I’ve also had a hormonal IUD for about 6 months.

Now today, I’m bleeding sort of heavy and have bad lower back pain and some cramping and also feel nauseous. It’s not possible that I’m pregnant again but I don’t know if this is my period returning or if there’s an issue with my IUD.

Side note, I’ve never been able to check my strings because I have a really high cervix and can’t reach them.

Thoughts?