Why Are My Menses Delayed After Taking Postinor 2?

hi doc, i took postinor 2 on the 11th of feb 2011, and started having a light flow 12th feb 2011 to 24th feb. Then had sex on the 19th of march and again took POSTINOR 2. I havent recieved my periods yet, upto today(24th feb 2011) and i really got worried. i did a pregnancy test and it turned out negative and the labaratory technician told me it could be too early to detect, i am really worried because my breasts are very sore, i am having abdominal cramps, and a back ache. i also have nausea. could i be pregnant?