Got an IUD Yesterday...A few question.?

I have a few question about my IUD. I get the Mirena IUD and i'm still cramping pretty bad. How long should I discern the cramping? It feels approaching period cramps on steriods.
Also my dactor told me not to hold sex for a week..is that for pregnancy prevention or is that so my cervix has time to close? Did anyone hold sex sooner or was told to hang about longer? I think you ought to read a bit nearly IUD's in nonspecific and about Mirena surrounded by particular:
http://www.netdoctor.co.uk/sex_relations...
http://www.drugs.com/pro/mirena.html