Is it possible that I could get pregnant if there was an ejaculation in me on March 6?

Hi, the last month I had on February 22, March 6, there was a relationship with my partner after a dozen hours I took a tablet the day after ellaOne ... I did not have any symptoms not desirable ... in March at the end of the month was very modest bleeding ... is already today 16 April I had just the same bleeding a few days over April 10th... done 4 pregnancy tests negative results of any signs of pregnancy did not notice if it is possible that I could get pregnant if it occurred march 6 to ejaculation in me ... the first time I took a tablet the day after that year I got as I should menstruation And now silence