TeenHealthFX understands your worries. It is not normal for a girl not to get her period after having sex. TeenHealthFX recommends that you go to a teen health clinic for an exam and a pregnancy test. A doctor at a teen health clinic can see you without your parent's permission and on a low/no cost basis. He or she can perform a pregnancy test to determine if you are pregnant. If so, the doctor will go over your options with you. They can then refer you to the appropriate place like an abortion clinic, adoption agency or an obstetrician..
If you are not pregnant, the teen health clinic staff can discuss birth control methods with you. Also, since you are now sexual active, you should go to a see a doctor anyway - preferably an adolescent medicine specialist or gynecologist - and discuss safer sex (condom use and a second method of birth control like Depo-Provera or birth control pills) to prevent unplanned pregnancy and sexually transmitted diseases.
If you do not feel comfortable with your family's doctor, local teen health centers or Planned Parenthood are good places to start. There you can receive anonymous care (means they will not need your parent's permission to see you) and on low/no cost basis. If you live in northern New Jersey you can call the Adolescent/Young Adult Center For Health at 973.971.6475 to make an appointment to see a doctor.