Took emergency pill on 3day of period it lasted for 10 day. now am late on my period did home pregnancy test its negative. what can be a problem?

Maybe: Emergency contraceptives such as Plan B can alter the menstrual for the cycle in which Plan B was taken, and sometimes the next cycle too. One should be sure to check a home pregnancy test about 3 weeks after sex, because Plan B works most of the time, but not all the time. If the pregnancy test is negative, one look forward to a usual cycle the next month (but start reliable contraception now).