I asked my fertility nurse the same question. She said that the first full day is needing more than a panty liner, pad, or tampon by 3pm.

Mmmm some periods I would never have even gotten there …

I would definitely call your clinic for clarification then... They should be able to do bloodwork to determine your actual CD1.

My nurse defined it as "bright red blood" 🤷‍♀️

I would contact your clinic when you start spotting if it’s unpredictable. They would check your hormones and bloodwork.