Nothing . It happens.
Yes, a period can last for as short as one day, in which case it is normal.
The date is calculated from the start day of the last menstrual period.
No it usually lasts for 5-10 days.
If you mean that your last period started on the third then the next one would be due on the 29th IF it remains at a 26 day cycle. Cycles are not always exactly the same from month to month. Start counting the days from the first day of your period. Don't include the first day of the next one when it comes as that is day 1 of your next cycle.
Neither, about a week after your period ends is normal.
most women have a 28 day cycle, you count from Day One when you start your period to get the date for your next period
Your period will normally come one day earlier. You better avoid sex until your period comes.
no you do not get a period if you are pregnant but that does not mean it is okay to have unprotected sex on your period sperm can live for 10 days
from the day it ended.
If you have finished with the pills and the faux pills, had your period, then you start the new set of pills after the last day of your period.
I think you go by when the sperm unites with the egg