14 day period post mirena?!

I had my Mirena removed 2 months ago and had one normal 7 day period. Now with this second period it has been extremely heavy and is still going and I am on day 14. I have called my doctor twice now, and the nurse told me that I need to give it a full three months post mirena for things to regulate. Has anyone else dealt with this or have any other advice as to what could be causing this?