Doctors on this site said in my case it's "very very unlikely" that i would be pregnant. took first response test day of expected period. neg. true?

You tell us: If your pregnancy tests are repeatedly negative then you are not pregnant. A pregnancy test can detect a bhcg of about 20 which means they are very sensitive and work really well. Trust the test. Repeat in a few days if you dont believe the result. If negative then you are not pregnant. Since you are 19 i then suggest you see a doctor to discuss birth control and avoid these scares.