I had two very early miscarriages that felt like what you describe. We had a hard time getting pregnant, and it was heartbreaking.

Doesn't an IUD work by causing miscarriage due to a foreign object being in the uterus? If that's the case, what you felt is probably not abnormal, hence the doc kind of brushing it off. Maybe the doc can prescribe you some heavy-duty pain relievers for those harsh ones?

(as I recall, they were horrible!)