Implantation bleeding 13 days after ovulation?

Louisa

I had my first positive opk on 23rd May, BD the next morning. AF was due on 4th June, didn't show. I am usually very regular. Very light brownish spotting on 6th and a little on the 7th too, still no AF. Four pregnancy tests all negative. What's going on?! Am I testing too early? Could this be implantation bleeding 13 days after ovulation? Anyone else experienced this? How long after implantation can you get BFP?