mirena and changing periods?

Alicia

so I've had the mirena iud since May of 2017 after my miscarriage. it helped regulate my always very heavy periods and helped with the severe cramping. in January I missed my period all together. this month I've gotten my period but it's light and all seems to be old blood. my question is, between regulating perfectly, then missing it, now light and old blood, can anyone explain why my period has gotten so wacky? Thanks ladies