Question

I had sex March 12. I ovulated March 16th he used protection and pulled out and we have one clear test from his vasectomy… but awaiting the time for a second one. I’m thinking slim chance of pregnancy. Now I’m cd 25 periods are anywhere typically between 28-34 days. And I got the tiniest bit of blood in my cm…. Could this be implantation bleeding? I took a test and it was negative but can help but worry