Needing help, kind of stupid question!

I have a stupid question….I have had PCOS for a long while now. Very irregular (but consistent) periods. Between 18-40 day cycles and varies from heavy to very lite. I am now doing fertility treatments and have to know when the first day of my period is, so I know when to start meds. I was supposed to start my period today, and have had very random spotting the last 3 days, Still no heavy flow. How do I know when to count my period actually starting? Anyone had a period with just a very lite spotting for a few days? It’s just so hard when your irregular. And it was confirmed that I ovulated last month, so I should have a actual period right?! Really appreciate any feedback!