When do you start counting the days as DPO?

Do you start with the first day of your fertile window or the last? According to eve my fw was the 9th-15th, and Glow shows it to be 8th-14th. We had sex on the 9th, 11th, 12th & 13th. I'm having pain on the right side of my pelvis and I think I might have had implantation bleeding last night but I am not sure when my "days past ovulation" start! Please help ladies! My husband and I have been trying for a year and a half, we desperately want to be pregnant! This possible implantation bleeding was on the 19th.