What Causes Late Period After Taking I Pill?

i had unprotected sex about 10 days before my periods and had an i-pill after this. my period was due for last week of october. i have a 28 days cycle. after consumption of the i-pill, i had slight periods for 2 days on the 3rd and 4th of november. till date i have not had any periods and since then, around 45 days have passed. i have had a home pregnancy test done once and the result was negative. what are the possible causes for this problem? please advise.