Am I sinful, and what must I do because of what happened out of ignorance?

Your act of circumcising the deceased miscarried fetus is an error stemming from ignorance of the Shariah ruling. You are required to repent and seek forgiveness for undertaking an action whose ruling you did not know. You are also required to explain this ruling to your family and inform those who participated with you of the necessity of repentance and seeking forgiveness.