Can someone give me some help me with this?

Im not yet tempting its my first cycle doing ovulation tests. It looks like the 17th was a true positibe but i felt O pain on the 19th by the 20 my test start showing negative so im truly confused because i dont exactly know when i should startcounting  as "DPO" so as of today i am either 6dpo or 4dpo (cause ive heard to not count the next day aftwr O as 1dpo so i would count the 3rd day as 1dpo) example If O day was the 19th it would make the 21st 1spo? Please help! Thank You!