Had Mirena for a year - just got my period for the first time in 4 months.

Hi Glow Users,
I had my Mirena inserted last September and haven't had any problems. It stopped my periods completely, which was great. 
However, last week I got my period for the first time in about 4 months. It was mostly brown (so old blood) and has been going on for 10 days now.  I can still feel the strings, no it's not dislodged.
Has anyone else experinced this? Having a long period after not having one for months?