How To Test For Pregnancy?

Hi...I had my period on february 7th last month...i had sex on the 13th... took postinor 2.... took both tablets at the same time...after I had my period again on the 21st... But it was light....then i took a ppregnancy test on the 6th it was negative...after which I had a 2day spotting ....but no period again ever since....could i be pregnant?????