Excessive cramping

I got my IUD at a young age. It was just what worked for me. I love having the IUD. I'm on mirena currently. I've had it for 3 going on 4 years. But, I have noticed that when its my "period" time I get terrible cramps. I've had it checked out and my doctor can't figure out why the cramps are so bad. Any one have any tips on how to deal with the cramps are what could be causing them?