Stopping nursing, going off the progesterone-only pill, and inserting Mirena can all lead to irregular staining. The adjustment period of staining on Mirena can itself sometimes last for months. Furthermore, stopping the progesterone-only pill is quite likely to cause a period, which will make you niddah.
The best strategy in your case would be to hold off on inserting the Mirena until you have been to the mikveh, and to continue taking the progesterone-only pill after the insertion. This is because, when you are tehorah, careful conduct with stains can often help you avoid prevent becoming niddah through staining alone. We recommend you read our articles “Stains” and “Toilet Paper” on this issue for more information and get back to us with any follow up questions.
When to stop taking the pills depends on how much bleeding you have from the Mirena. If you become niddah from the Mirena, then also stop the pill. If not, stop when you have had enough days after mikveh that you are emotionally prepared to possibly become niddah again. You also might get another period, even on the pills, as you did this month.
Please also note that bleeding within a day of Mirena insertion can sometimes be considered dam makkah from the procedure; please see our article for details.
Please don’t hesitate to write back with any further questions.