picked a handful of violets in my name

IUDs are tricky things. I had bad cramps only for about 6 hours last time, but was quite sensitive for about five days and had a longer than usual period. Here, they insert the IUD while you're bleeding, since your cervix is more relaxed, so I don't know if you'd respond differently at different times in your cycle. I think you're still within the range of normal-esque cramping, though. If it lasts for much longer, perhaps your uterus is too small for the size of device and you're getting cramping from that? Just an idea.