I don't have an explaination, but that just happened to me today. I've had the Mirena IUD for a little over a year now and my last Dr exam a couple of months back was normal. I usually have pain during intercourse for the first few minutes then it goes away. I also almost always have light bleeding after intercourse as well. It's not too much, but enough to turn the mess pink.
Today while at work, I felt a little wet. I didn't think much of it because ever since I had the IUD inserted, now I have to wear panty liners all the time; so I thought it was just another mucus discharge (which is normal, by the way). When I was leaving, i went to the restroom and my entire panty liner was bloody with lots of blood clots. it was heavy enough to soak through my pants. I had no pain, no warning, nothing. About a month back I remember having sharp pains ono the right side of my abdomen. They would come and go whenever. it could have just been gas ( I actually went to the hospital one time thinking a gas pain was something seriously wrong) again.
After frantically searching the internet for about an hour today, the only thing I can come up with is an ovarian cyst. from what I understand, Ovarian cysts are normal in IUD users. However, when one pops and causes bleeding, pain would be a factor too. No pain for me. if i'm still bleeding tomorrow I will call my doctor and make an appointment.
I will keep you updated. If you find out anything as well, please keep me posted.