Every month doesn't have the same number of days in it. :>) If you have, for example, a typical 28 day cycle . . . it's not going to come on the same day of the month every calendar month. It's better to count how many days starting with the first day of your period being day one and count from there. You should notice a bit of a pattern but we can always have an off cycle out of nowhere. MyCycles, the app here at Med Help is good and you can find it in the above tool bar under health tools to download for free onto your phone or tablet.
Anyway, if you have had unprotected sex though and your period doesn't show when next due, then take a test. good luck