Severe cramps that feel different from what you normally experience are worth taking seriously, especially since you are on an IUD, as there are a few possible things going on and it is hard to say without a proper check. I would not wait too long on this one, it might be good to see your PD or a gynaecologist soon, just to rule out anything like IUD displacement or other issues. Getting it checked in person gives you a clearer picture than anything online can.