It could be that you implanted late or it's just taking a little longer for hcg to build up. Some women don't test positive til they're a few days late.
But it could be stress. It could be you ovulated a little late (the luteal phase tends to stay the same so if you ovulate late, AF will come late). It's hard to say. I was a few days late my first month TTC, and I'm sure it was just the combination of stress and psyching myself out. I don't mean to be a downer, I just know how stressful that wait was and how disappointed I was when AF finally came and I want to be realistic. I'm hoping for the best for you!