According to my calculations, my period is late. i've just started to keep track of when my periods are suppose to be happening. every 28 days right?

Cycle length: Cycle length can vary; most normal ones are between 25-35 days in length. If you are just starting to monitor, follow for a few months to see if cycle is always around the same number of days.

It varies: It varies among women and sometimes in one woman, depending on stress, weight, and other factors. Anywhere from 21 days to 35 days is considered normal.