What Causes Prolonged Bleeding After Having Mirena Coil Fitted?

Hi, I had a mirena coil fitted on the 16December 2014 and probably had a window of about 8 days clear of bleeding. This week the coil fell out and the clots and bleeding so bad that I went o late night GP who prescribed 5 mg northeristone. How long should I stay bleeding for on these now as still quite heavy!