I am always amazed the lack of details in questions like these. How old are you? Did you use birth control when you had sex?
Young girls typically experience irregular periods. They usually straighten out in their twenties.
If you had unprotected sex, there is ALWAYS the possibility you are pregnant. There's ALWAYS the possibility you contracted an STD unless a condom was used.
Unless you want a baby right now, I would suggest that you and your sexual partner practice safe sex EVERY single time. In the meantime, take a pregnancy test - mornings are best - to see if it comes up positive or negative. If it's negative and you still don't have your period in a couple of weeks, test again and then hustle in to your doctor for confirmation.