You can take a pregnancy two weeks (preferably three) after you had sex.
Your period isn't very late yet, and you're only 15, so it could just be due to random abnormalities. However, since you had unprotected sex (even if it was just a few seconds) there is a slight chance of pregnancy, so I would suggest taking a test. No, you would not notice any symptoms necessarily- the only way you'll know if you're pregnant is if you take a test.