Dpo? Confusing lol.

Tara • Pregnant with our 3rd baby!
Everyone is discussing Dpo. I'm new to this. Do you start counting after your fertile window is closed or on the day you believed you o'd? My fertile window ended on a Thursday, but I had strong cramping sensations on that Tuesday. So when do I start counting? Either way I'll be waiting til my af is late. Thanks for reading :)