IUD Pain

I got the skyla IUD put in last Thursday (22), prior to this I was on the pill for almost 6 years. I'm still having some cramping and spotting. When I got the IUD the doctor said I have a higher chance of rejection because I have a small uterus, so I'm worried that could be the cause, but I tend to worry too much. I was wondering if I should be worried and what your experiences were with getting an IUD. How long did your cramps last?