What Causes Heavy Vaginal Bleeding While On Mirena Coil?

I have had mirena in since oct of 2009, my periods were always irregular coming when they want and usually only lasting a few days, not even enough to use a tampon. About 2 weeks ago i started bleeding heavily and the blood was almost black and this has been happening for almost 2 weeks, it has lightened up but its still going on, what could be the problem?