Buy a pregnancy test & take it to be sure. It is 99% effective like the birth control pill. Although we may depend on them as if they are 100%, that 1% is still there for a reason. If your pregnancy test comes back negative, perhaps it is side effects from beginning the Mirena. Whenever you start it, it can have similar effects on the body as your hormones get used to it.
If they don't subside, call your doctor and speak with them about your symptoms. You can always take another pregnancy test to confirm your results before calling them.