Abdominal pain for the last 4 days now and I am spotting dark reddish brown blood I have an IUD in but I'm in severe pain that is now in my back also

Need to be seen. Several diagnoses need to be excluded. One life-threatening possibility is an ectopic pregnancy. Iuds prevent pregnancies but in the rare cases where pregnancies do occur, the chances of an ectopic pregnancy is higher with an iud. Pelvic inflammatory disease could also explain your symptoms. This needs to be treated in order to prevent infertility. You should be seen asap.