Could i be pregnant ?...

Ive been taking birth control for about 5 months now , never missed a day and i am sexually active , even though im sexually active ive always gotten my period... However december when my period was suppose to come i had excruciating cramps and instead of blood a brownish discharge which i thought was just the start of my period ... It only lasted about 3 days and after that i never got my period ..... I took 2 pregnancy test and both came out negative ..... What could be the case for this ?