I'm 19 years old. I received my period on August 19th and got off on the 26th. On August 27th and 28th I had sex (protected). My period for this month was scheduled for the 19th and I still haven't received it. I'm having sharp pains in my lower stomach, back and nausea. I took one pregnancy test Sunday and another today and both came back negative. Could it be I'm pregnant and it isn't showing or is my period coming? Please help, I'm super nervous.