M.B.
I had regular VERY light, short periods while on Mirena for a year and a half. They were regular after I had it removed, too. If I remember correctly it took a month or so to get back to "normal". I could tell you the exact day and time I would start to flow before, during, and after Mirena. I think that your body is just trying to adjust to the lack of hormones.
I will tell you this, though, post Mirena periods were HORRIBLE for me at first. I am not sure if I just didn't remember what periods felt like after being pregnant, but OWIE! They hurt really bad at first and were VERY, VERY heavy.
I would wait a couple more weeks. I bet it will show up and things will get back to normal. Unless you think that you might be pregnant!