First of all stay relaxed. I hope this is spotting, not bleeding. The reasons may be- implantation bleeding due to pregnancy. The second possibility is sometimes progesterone can cause spotting (thus is a known side effect of progesterone). The third possibility is rare, provided IUI is done in sterile set up- that is infection after IUI. Right now pkease stay relaxed, go for urine
pregnancy test after 5-6 days. All the best.