It depends when you ovulate. I ovulate as early as day 10 so if you ovulate that early then there is a chance due to sperm survival time as mentioned above. Would it be more likely your normal.ovulation was delayed due to stress/illness this month? That would cause late af. Bottom line is, if you've had unprotected sex there's a possibility you could ve pregnant. Take a test!
Sounds unlikely you're pregnant. More likely to be either delayed ovulation or anovulatory. Most woman have a couple of anovulatory cycles a year. I had one a couple of cycles ago, my af was several days left. If af doesn't arrive, I'd test again in a few days.