What Causes A Delayed Menstrual Period With Negative Pregnancy Tests?

I'm 19 years old and just recently lost my virginity on march 14th and then did it again on April 4th and used a condom and pulled out both times. we saw no tear. But my last period was on March 24th and I havent had one yet(not even symptoms). I had very slight brown discharge two weeks after the second time that lasted a week and then when i was supposed to get my period, one day i had stringy dark brown discharge but only for a day. I've taken two easy blue digital tests and both were negative. I even took the second one in the morning to get more accurate results. I'm not wanting a baby yet, so i'm getting really scared about missing my period. any help would be appreciated. thanks.