I'm on birth control an i had unprotected sex the day befor i was suppose to start. i was 3 days late then had a heavy period. can i be pregnant?

Unlikely, but...: A urine pregnancy test, done correctly, gives a reliable result 3 weeks after sex (3 weeks after when conception might have occurred), or 1 week after missing a withdrawal period. If done sooner, a positive result is likely real, but a negative result may be due to testing too soon, and thus lead to a repeat test. If a woman gets her withdrawal periods, then she is not pregnant.