What Does Missed Periods With HPT Negative Mean?

Hello, I'm a 17 year old girl. I don't remember when i got my last period. I'm really scared. I know i'm not pregnant since i took a pregnancy test the other day. I do have sex, but with cautions. I decided to talk my mom into putting me on Birth control, so i'm suppose to wait until the sunday after my period. But i didn't get it yet. please help me!