What Causes Brown Vaginal Spotting While On Mirena?

I have had Mirena for about 8 months now. In the beginning I had the typical light bleeding for 2 months then my period and adjusted to normal 3-4 days light bleeding cycle. currently my period was dark brown and lasted for 2 weeks although it was very light flow. Two weeks later the dark brown bleeding started again. Should I be conncern and how can I stop the spotting?