Ovidrel contains hCG that helps mature the follicles and trigger the release of the ovum or ova. If this were combined with IUI, the IUI itself can sometimes cause cramping. Other options include being hypersensitive, hormonal changes caused by the ovidrel that are affecting your uterus, etc.
Don't take this as a sign of whether you are pregnant or not. It can go either way :-)
Rachel Inbar