What Causes Prolonged Periods After Taking Next Choice Pill?

I took Next Choice in Feb. i bled around Feb 13th and it continued for 5 days. after that I had not come on my period for the entire month of March. I took 3 pregnancy tests and they all said negative. Thursday March 31st my boyfriend ejaculated in me and I took Next Choice again within 2 hours after sex, the next day he ejaculated in me again... am I now liable to be pregnant because of this?