There is a foreign physical device inside your uterus, it sounds reasonable that the contractions of uterine muscles are squeezing it and may cause more pain.
I would assume that the position and angle of your uterus and/or IUD also matters. If you have the copper IUD, then your body could be reacting to the metal and/or the increased amount of prostaglandins.
If it's still severe after a few months or you experience other symptoms also, I would recommend talking to your doctor.