Period

Your GP can help you and is bound by confidentiality, so your parents don't need to find out.

Personally, I think a girl your age is still quite young to have sex; I would have waited a few more years myself. But that doesn't help you now.

A few questions: Are you on the pill? If so, have you forgotten to take it? If you take it regularly, the chance that you're pregnant is minimal.

Are you not on the pill: Is your period regular, or does it sometimes vary by a few days? Especially if you haven't been menstruating for very long, it's quite irregular at first, which is normal and doesn't necessarily mean you're pregnant even if you use contraception. Your ovulation just isn't on a fixed day of the month yet, so the withdrawal bleed is also later.

Did you use a condom during sex? That's not only important to prevent pregnancy but also to avoid getting an STI.

Actually, you should talk to your parents; they can help you too. But if you don't feel comfortable doing that, go to your GP.