Mirena iud

Ladies this is my first time using the iud. Before id have a moderate to heavy flow lasting 7 to 8 days. With a 28 day cycle. My first period after insert started on time with a moderate to light flow lasting 13 days. That being said. Im now late on my 28 day cycle from when my period started. Is this because my period lasted longer then normal? Should i be worried or just count 28 days from when period ended.