What Could Cause Delay In Periods?

Hi,I am a 21year old girl from china.I had my period on dec28th,2010 and again on feb 2nd,2011.After that i had sex with my BF on feb26th ,2011. I took a pill (contraceptive :here in china is Mifeprostone 10mg x 1) within 5hrs and again had sex on feb27th in which he didnot ejaculate inside(but i am not sure). I had Brown discharge(a very few quantity) on Feb28th.I dint have periods till now.I took a HPT (hcg urine test former with sensitivity 25mUI and later with 10mUI) on march 10th and again on March 15th.Both of them turned out negative.I am so worried of my periods.Am i pregnant?