I was post menopausal when I had mine fitted and was told to expect spotting for around 12 weeks, I had spotting, albeit decreasing over time, for almost exactly 3 months.
I was given a little patient information booklet which contains useful info, this says that if you have Mirena fitted for heavy menstrual bleeding "you should have lighter periods after 3 to 6 months" And for "all uses.......you may have lighter periods or painful periods or some spotting and irregular bleeding during the first few months after Mirena is fitted. You may have prolonged or heavy bleeding or an increase in the frequency of bleeding, usually in first 2 to 3 months, before a reduction in blood loss is acheived"
So it sounds as if you are 'normal' and probably backs up why your checkup has been scheduled for about 6mths after insertion. But you could always check with your GP/clinic if you're worried.
Suzie