I'm a teenager.my last period started on 28th august, 2012.i have had sex only once and even used a condom.i took 4 pregnancy test which came negative.?

Doubt you're pregnan: There are lots of reasons why teens might miss a period(s) (pregnancy, stress, weight, exercise, how long you have been having periods) not all preg tests are the same. Some will turn positive when you've been preg only 7 days and others not until you've been preg 3 wks. If last period was nl & no sex since you're prob not preg. The info you need exceeds allowed 400 characters, so call your doc.