That happens sometimes, escpecially if you're still young. And if you don't get your period on the 20th, WAIT until the 27th until you buy a preganncy test. Periods can be delayed by a couple days. When I was your age, I was ALWAYS a week late. Its only the last year I've been regular.
And DO NOT look for symptoms of pregnancy. Symptoms can be related to so many things. On top of that, you'll think to much about it and get your body believing that you're pregnant.