You may not be pregnant, I'd wait at least a wk then go to your doctor and tell them your late and get a test done.
I was 3 weeks late once it just wouldn't come I'd been to the doctor twice and she said I wasn't pregnant and the day after she got tests organized to make sure everything was ok I got it.
If you know it may happen again go on the pill, its better to be safer then sorry. But Honestly you should use condoms until you've been with your partner for a long time and you both get medically checked out. You never know what you can catch.
I know every1 thinks it'll never happen to me but the odds aren't in your favor.