Hormonal IUD pain/bleeding

I’ve had the mirena IUD for about a year and a half now and didn’t get a period or any cramps for the first year. Then after having sex (not the first time since I got it) I started bleeding one day and having unimaginable cramps. It stopped after a couple days so I disregarded it but it’s started up again recently every few months. The blood has been really mucosy as well. I’ve been to the doctor and had a pelvic exam and an ultrasound and they said everything looks normal and it’s still in place so I’m wondering if this is normal or if anyone else has experienced this?