Did you have sex earlier in the month as well? Or was the day before your period the only time? Your cycle is exactly that, a cycle. There is a lot that goes on in our bodies through the month. You Get your period and that's cycle day 1, then from there your body prepares an egg for maturity. Which textbook is 14 days later, but nobody is textbook. So it can be anytime from cycle day 10. After your egg is released, you have up to 36 hours for that egg to be fertilized. Some are less and some may be more though. If the egg is fertilized it will take about a week to two weeks for it to travel to your uterus where it will implant. If it isn't fertilized then the egg dies and when implantation doesn't happen your hormones will let your uterus know that it isn't pregnant so that when it sheds the lining known as your period. There is a very very small window after the egg is released for it to be fertilized. Odds are that you are not pregnant. And if you were, and your period is really that late, then you would be getting a positive pregnancy test. You really can't know if your period is late without knowing when you ovulated, because you can ovulate later or earlier than you normally do. Our bodies are amazing and crazy at the same time.