If you were supposed to start on the 11th, generally you would have ovulated around the 28th of August and thus would not have conceived by having sex on Sept 3rd. However, there are cases where women actually get pregnant from having sex during their period. Ovulation usually occurs midway through your cycle but can happen at any time.
Once conception takes place, it usually takes between a week and two weeks for the baby to implant in your uterus. Once that happens, your body starts having higher levels of the hormone to give positive pregnancy test results.
If you did conceive on the 3rd, the baby could have implanted by the 10th or so and thus you could have a positive test on the 12th.
You can get a positive as early as 8 days past ovulation. If you ovulated the day before or the day you had sex, than it could have been 9 or 10 days past ovulation for you.
did you have sex any other times before that?
If your period was due the 11th, then its not too soon! Most pregnancy tests are accurate a few days before your period is due, and all are pretty much accurate after you're due. Congratulations!