J.,
Not an IUD user, myself, but I did consider it. A couple of things that really stood out to me about what you said.
You seem to be really concerned about potential damage to your body. You may be especially sensitive because you have already had a very invasive experience with your reproductive system, which is completely understandable.
I say this because, sometimes, even though IUDs may be as safe as anything kind of birth control device, and for many, many women a very desirable and effective method of B/C, it may not be right for you. In reading this, I wondered if your fears and concerns are causing extra tension in your uterus, and thus making the cramping feel more severe.
You also mentioned that you are breastfeeding. (Good for you!) When a woman breastfeeds (as you may already know), she releases the hormone Oxytocin, which is the same hormone that is released during labor, and also released during orgasms. Oxytocin causes your uterus to contract, and for some women, that might cause added cramping, especially as your body is adjusting to your IUD.
If I were in your shoes, I would really think about your comfort level with the IUD, now that it's in. If it is causing you anxiety, it is probably not the right method for you. However, it may also be a simple issue of adjusting to this new "thing" inside you. Listen to what your body is telling you and you'll know what you need to do :)
-R.