Confused About My Menstrual Cycle

Now let me explain... I have been on an IUD since giving birth to my son July 2015. I have not had any cycles since then. About a week ago out of nowhere I start bleeding, but I only bled for 3 days. The first day of bleeding was light, so I just used a pad. The second day my flow was heavier to the point I thought a tampon would be better. But I remembered my IUD and I decided to check for the strings. I could feel my IUD partially dislodged! Well, for the rest of that second day I left the IUD the way it was and used pads (I hate pads!). Anyway, day 3 I'm still bleeding and the dislodged IUD was getting uncomfortable so I removed it the rest of the way. I bled the rest of that day, and then just all of a sudden I stopped. It's been a week now since the incident. I have had some pain in my pelvic region and have been nauseous. I'm trying to make an appointment with my primary care doc, but work scheduling is getting in the way. But, bottomline, how have I gone over 2 years period free thanks to Mirena to all of a sudden randomly bleed for 3 days?? Note: prior to pregnancy I had very normal, like clockwork periods. My only complaint then was that they were always annoyingly heavy.