I'm assuming you have Mirena, the progesterone-impregnated kind that thins the walls of the uterus so you stop "menstruating"? you still ovulate, but the lining of the uterus doesn't get thick so you don't have the shedding.

I, too, have an IUD, but I don't think it would make any odds if I did or not with my cramping. I found that the older I get (I'm 36), the more pronounced my symptoms are for both pms AND when I ovulate. When I ovulate, I can have a dull ache for a couple of days with maybe half a day of ache enough for me to reach for ibuprophen sometimes.

I started to use Hyland's PMS (homeopathic) and I find that it does help, though I wouldn't say it's some miracle that'll take the ache completely away.

If you keep track of your cycle, it should happen around day 14 for these "cramps", if it's not at around 14 or 28th day, I would get it check out to make sure that the iud's still in place.