Don't know if I'm

I had sex last month and I'm not sure if it was during my fertile window. I missed my period entirely; I took a pregnancy test about a week after my period was due but it came out negative. My next period was supposed to come 4 days ago but still no signs of it. Not even the feeling of it coming. But I don't feel as though I could be pregnant either; I have no signs of it. Please help?
P.S. I'm only 16. I can't go to the doctor for a blood test because my parents can't know about this..they still think I'm a virgin. They'd kill me if they knew otherwise. If I do turn out to be pregnant I would need an abortion but I don't know how to do it without them knowing and I need the money. Please give me advice about this too. Thanks.