I currently have a Mirena--my first since my second son was born back in September, but my second overall (I had one for a few years between the births of my two sons). I've never had something occur as you have described, but I know I had quite a bit of bleeding when my latest Mirena was inserted because I had it not long after my son was born. For some reason, the lining of the uterus becomes even more irritated from the equipment used to help insert the Mirena so soon postpartum. Maybe you had a clot that finally worked itself loose? As the others said, if it had been a pregnancy, your HCG levels (the pregnancy hormones) would already be dropping quickly. I think the best way to know for sure is to get a blood test from your doctor. I'm not sure if a home pregnancy test would work--I don't know if they're that sensitive. Good luck, and I hope you find the answers you need.