What Causes On And Off Spotting Without Full Periods?

i had unprotected sex on 26 dec 2013 ( 9 days after my periods). took i pill within 2 hours. following which i had brown discharge around 7/8th jan. lasted for a day or two. again had discharge for less than a day around 18jan. Took pregnancy test last week . it was negative. still no full period. am i pregnant? if not, when are my periods expected?