I've Had Mirena For About Three Months And Have Had

I've had Mirena for about three months and have had normal period the whole time. About a month ago I got my period like normal, except this time instead of tapering off and being gone within 6-7 days, my period stayed, very heavy, for about 2 weeks with cramping (I never get cramps). It has been about 2 weeks since having my period and I had unprotected sex. I took Plan B just in case something was wrong with Mirena since I had all of those strange symtptoms. The strings are still in place.. Is the longterm bleeding and cramping thats unordinary typical of the Mirena device? or could my Mirena have shifted