Iud, spotting, cramping, severe back pain, both breasts extremely sore swollen and leaking a milky substance. neg preg test. should i be worried?

Worried no, but...: Need to worry? no. IUD's can and do fail to prevent implantation resulting in ectopic pregnancies.(outside of the uterus) which can be problematic. At the very least you may have a prolactin secreting growth which may account for the leaking breasts if you are not taking other drugs that can do that as well. Obtain a prolactin and serum pregnancy test at your MD's office to clear up the concerns.