What Causes Delay In Periods And Pain In Nipples?

i took the pill the month of december and when i ended them i had my regular period on the 31st of december. i did not take any type of pill on january nor had any type of sexual intercourse of any kind. My period was scheduled for the 28th of january and it is the 1st of february and nothing yet. i show some premenstrual symptoms such as blooding and my nipples hurt. should i be worried? what may be the causes of being late?