I've had mirena for about 6 months now. I had it put in due to ve...

I've had mirena for about 6 months now. I had it put in due to very heavy periods, and i must say for that reason the mirena is GREAT. My sister has had the mirena for 2 1/2 years now and a few times she had cramps/severe pain so bad she couldn't even get to a phone, one time she actually fainted. Well i experienced my first "cramp attack" (as we call it) and it was bad, but my second experience was last night, and it was crazy. It hurt so bad i fell to floor in pain. It felt like a constant contraction that wouldn't go away for about 20 mins. My sister has been to the doctor several times for it and the scary part is they don't know what it is. We know its from the mirena though because we both have it. Has anyone else experienced this?