I got pregnant with the mirena with my 2nd child. The mirena had fallen out of place so it wasn't working.
I got pregnant with the Mirena IUD, thankfully it was early enough to remove it "safely" Definitely get it checked as soon as possible.
It is very possible to get pregnant on the Mirena IUD it happened to me about 9 1/2 months ago and for some reason that's what it did to me I would get positives and negatives. You definitely need to go see your doctor.
I would make an appointment to be seen. Pregnancy on the IUD is usually uncommon but if you get pregnant during use of that it "can" be life threatening, be an ectopic pregnancy, etc. Make an appointment and let them know.