How to naturally jump start a period..?

So my period is 3 days late. I'm not pregnant. My husband is very fresh out of chemo and sterile, but I've tested a couple times to be 100% sure. BFN. We're supposed to start IUI's with donor sperm this cycle but of course my period is late for like the first time ever. I've been tracking my cycles to stay up to date. I'm currently 17 dpo. Had brown spotting at 11dpo and was sure my period was about to be coming in a couple days.. NOPE. Any ideas how to naturally get nature to move along?