Going off of hormonal birth control can cause your body to be out of whack for a while. It sounds like you got your period after you had your Mirena removed, and you are now waiting for your second one, correct?
As I mentioned, it is very normal to have whacky cycles after coming off of BC, so I would definitely say that you don't really need to start worrying yet. If you go 60 days without a period (or a BFP), call your doctor to get your period jump-started with medication.
I would highly suggest you read the newbie link at the top of the page - it is full of great information. In addition, the website Fertility Friend has great tools for charting your BBT, CM, and other fertility indicators. They have great tutorials that help you understand your fertility better.