Implantation Pinching 12dpo

Ca

If the pain / Sharp Pinching I felt yesterday at 12dpo was indeed implantation occuring (it was sharp pinching on the left side of my abdomen and it felt pretty dang deep)..... when would I be able to test and get a BFP? I testing this morning with a First Response and it was BFN. I am still having the deeper pinching feeling today along with some minimal cramps. AF due to arrive tomorrow... HALP lol