When to start counting DPO?

Hello! I was just curious if there is any general idea as to when to start counting DPO? I didn't temp this month so I am really unsure. I did use opk's. I normally test twice a day. On cd14 I tested at 12pm and got a negative result. Later on I started cramping on my left side so I tested again at 4:45pm and got a positive result. I tested again at 9:45pm and it was negative again and has been ever since. I am currently on cd18. I had severe cramps the day of my positive result all the way into the next day. What day would be 1dpo?