The initial spotting that you exerienced was probably from irritation to the cervix or uterus from whatever may have caused your IUD to fall out. The bleeding that you are experiencing now is either your natural period, bleeding caused by the withdrawal from the progesterone, or both. The fact that you are having headaches as well, I am thinking that you are at the point where you would have naturally had a cycle anyway. Most likely, the symptoms are made worse by the withdrawal from progesterone.