If your last period began on 6th May and you had sex without a condom on 31st May, you are unlikely to be pregnant, although it is not impossible.
Last period, sex, and the chances of becoming pregnant
The time when you are ovulating and therefore most likely to become pregnant is around two weeks after the start of your period, or halfway through an average menstrual cycle. That would put your ovulation (your most fertile) time at around the 20th - 23rd May.
What you should do if you're scared you might be pregnant
This is the time when your egg is sitting ripe and ready, waiting to be fertilised. You can learn more about your body and conception here:
http://www.webmd.com/baby/guide/understanding-conception
However, you may have your dates wrong, or you may have an unusual cycle. If you believe yourself to be pregnant, take a pregnancy test and seek medical advice. American teens who'd like to speak to someone about pregnancy can go here:
http://www.teenhealthandwellness.com/static/hotlines#TeenPregnancy
British teens can go here:
www.nhs.uk/conditions/pregnancy-and-baby/pages/teenager-preg