What Causes A Delayed Period With Negative Pregnancy Test?

I am 20 yrs old female and had sex with my friend on 24th day of my periods and now my periods are delayed for one week and have tested home pregenecy test at home it showed negative am i pregenent? as i am not getting my periods i am worried as i am a student and cant tell at home to parents. if I am pregenent is safe secret abortion possible now doctor plz guide me. and what it will cost me.