I also have these sort of issues. I have mine for a bit over 2 years and for 1 and a half of that I have been having issues, not just the pains but they are really horrible. With me it gives me lots of stabbing pains and cramps in my uterus and the left side of my stomach. I don't know what it is but I am going to get mine removed because of this.
The mirena can cause pain if it has shifted positions. If the pain persists for more than a couple of days and you're not just having menstrual cramps then you need to go see your gyno for an ultrasound to check the position of the mirena.
What type of birth control do you have? Were is your pain?